Transcript for:
Understanding Prokaryotic Cell Structure

this is the lecture for chapter four for prokaryotic cell structure and function there are three general types of microbes that we are going to cover in this class the prokaryotes the eukaryotes and the viruses and this chapter is all about prokaryotes so before we get into any details about the prokaryotes i want to go through the word prokaryote so the first thing that happened during the history of microbiology is the development of the microscope by hook and then von luenhook refined the microscope and that's when they started looking at living cells and when they first started looking at the living cells they noticed that there seemed to be two types of cells that they would see they would see very large cells that had this dark kernel in them they called it a kernel and then they would see little cells that did not seem to have that kernel in them so at that time what they thought would happen is that these little cells would eventually mature into the larger cells grow into larger cells that had that kernel so they called these little cells prokaryotes pro means before karyo means kernel so these were called the prokaryotic cells and they thought that eventually they would grow mature into the true kernel cells which are the u karyotic cells eukaryotes u means true karyo means kernel so that's what they thought at the time but now of course we know this is not the case these small prokaryotic cells are a completely different type of organism compared to the eukaryotic cells so this is not the case prokaryotes do not evolve into eukaryotes but back when microscopes were first being developed they didn't know that of course and in the eukaryotic cell what is this kernel that we're able to see in most eukaryotic cells it's it's the nucleus the next thing i want to do are go through some general characteristics for prokaryotic organisms the first defining characteristic is that they are very very small much smaller than eukaryotic cells the second characteristic is that they have no organelles compared to the eukaryotic cells which are much more complex and do have organelles next all prokaryotic organisms are unicellular that means one individual organism is composed of only one cell also prokaryotes can be either heterotrophic or autotrophic and this refers to their type of metabolism hetero means other trophic means feeder so if an organism is heterotrophic that means it must ingest other organisms in order to get a source of carbon and energy prokaryotic prokaryotic organisms can be heterotrophic or they can be autotrophic auto is self feeder so an autotrophic organism is able to produce its own or form its own organic carbon from a process like photosynthesis the last characteristic that prokaryotic organisms have is that they have asexual reproduction prokaryotic organisms are divided into two general groups so it's divided into the bacteria and the rke and even within the bacteria often the bacteria divided into gram-positive bacteria and gram-negative bacteria the first prokaryotic group i want to go through are the archaea our ke is the first prokaryotic group that i would like to go through and looking at the word archaea refers to ancient because it is believed that some types of archaea may have been one of the first organisms on earth in terms of structure the rk have all the prokaryotic characteristics like being very small no organelles unicellular heterotrophic and some are autotrophic and also reproduce asexually one of the things that differentiates the archaea from the regular bacteria is that the archaea do not have peptidoglycan in their cell walls and when i talk about the bacteria i'll be talking about peptidoglycan a lot but arcade do not have any peptidoglycan in their cell walls and another difference is that most are ke are extremophiles extremophiles file means to love they prefer extreme environments some of the rk are hello files hello means salt file means to love so they love very salty environments like the salt lake the great salt lake in by salt lake city others are thermophiles that would live here thermo refers to heat so they are heat loving they can live in hot springs and there are also acidophiles that prefer very acidic environments so the archaea live in these extreme environments some bacteria can live in extreme environments but many bacteria do not they live in more mild environments so the difference between the archaea and the bacteria is that archaea do not have peptidoglycan in their cell walls and they also all live in extreme environments for the rest of the lecture i'm going to be talking about bacteria and actually for the rest of the semester whenever i refer to prokaryotes i am referring to bacteria identification is going to be an extremely important theme in the whole semester during many lectures i will talk about the importance of being able to identify different types of bacteria and in fact your unknown project which which is due at the end of the semester is all about identifying a sample of bacteria that you have been given and one of the important characteristics that you need to know when trying to identify a strain of bacteria is the type of shape its cells have for but the vast majority of bacteria the bacteria have only three basic shapes caucus which is a rounded shape bacillus which is a rod shape and then spiral so for the most part the bacteria come in only three options for shape coccus is singular cocci is plural so either a rounded shape or a bacillus shaped bacillus is singular bacilli is plural or a spiral shaped now these shapes are determined by genes so the shape is species specific what that means is each species has only one shape so they're monomorphic one shape so for instance caucus means brown circular shape staphylococcus epidermidis staphylococcus it tells you that the shape of those bacteria are round e coli is a bacillus so all members of e coli will have a bacillus shape and another species treponema pallidium that is a spiral in addition to shape bacteria can also have specific arrangements so bacteria that come in the shape of a caucus or pleural cocci there are five different arrangements that those bacteria can be in and the arrangement has to do with how the bacterial cells divide when one cell divides into two it's how those cells remain attached together so the different arrangements for the cocci are first diplococci diplococci diplo means two so that's where you look the cells under the microscope and you can see pairs of cells streptococci strepto means strap so they're in long chains then there is tetrad so tetrad 4 that's where you see groups of four cocci bacterial cells bound together then there's sarcinae sarcinae is when they are in groups of eight and then the last one staphylococci staphylo refers to irregular clusters so that would look like a cluster of grapes and again the arrangement is also dependent on genetics so for instance staphylococcus bacteria they are named that because they have a staphylo arrangement and streptococcus bacteria have a streptoarrangement and that's another thing also when you are describing the shape of the bacteria shape and arrangement it can be lower case than in regular type but if you are referring to staphylococcus epidermidis then it needs to be capitalized and italicized if it's a genus name bacilli also have their own arrangements so bacilli is plural bacillus is singular so they can be single bacilli they can be diplobacilli again diplo means two so they are in pairs they can be streptobacilli so in long chains and you can also have coco bacilli which are more like rounded rods spiral bacteria don't really have different arrangements but there are three different types of spiral bacteria a vibrio is like a curved rod so sometimes it's difficult under the microscope to determine if a bacterium is a diplo bacillus or a vibrio but a vibrio is a curved rod a spirulium has several curves to it but it's very rigid compared to a spirochete which has several turns to it but it's flexible so it may not be as regular with the curves there are a few bacteria that have unusual shapes and there are some star-shaped bacteria some rectangular bacteria but these are very unusual bacteria and they do not usually cause disease in humans so when we're talking about the bacteria that cause diseases which is what we focus on mainly in this class all those bacteria are going to fall into one of the three groups caucus bacillus or spiral i'm going to spend the rest of the lecture going through prokaryotic cell structure in more detail and again from this point on when i say prokaryotic i am referring specifically to the bacteria and as i go through explaining these different structures there are three things i want you to keep in mind the first thing is how prokaryotes compare to eukaryotes so as i go through the different structures i will point out differences between prokaryotic cell structure and eukaryotic cell structure and some of it may not make sense until i also have the lectures on eukaryotes too second thing i want you to keep in mind is how the different structures are used for identification i've already mentioned that shape and arrangement can help with identification but identifying the different strains of bacteria is very important especially in medicine the third thing i want you to keep in mind are how those structures affect pathogenicity how those structures make a bacterium more likely to cause disease so it can make them more pathogenic they cause more damage or it can make them harder to kill so these are the three things i want you to focus on and think about as i go through the different parts of the prokaryotic cell structure this is a micrograph of atypical bacterium this is obviously a bacillus type of bacteria because it has a rod-like structure and this just shows basically what it looks like under a microscope and this is a cartoon version of a bacillus bacterium and this has all the different structures that i'm going to go through in the rest of this lecture and the way i'm going to go through them is i'm going to start from the outside of the cell and move inside to the cell so the first thing i'm going to talk about are the appendages and then after i talk about the appendages i will go through the layers of the cell and after i've gone through the layers of the cell then i will talk about what you find inside the cell so appendages prokaryotic cells may have three different types of appendages they could possibly have flagella they could have fimbriae and they could have a pillis flagella are appendages that extend out from the cell and they are relatively long compared to the length of the cell so they are relatively long and thin and for the prokaryotes only the bacillus and the spiral types of cells can have flagella caucus-shaped cells do not have flagella they never have flagella and if you compare the structure of prokaryotic flagella to eukaryotic flagella the prokaryotic flagella are much more simple in structure compared to the eukaryotic flagella now one of the things about the flagella prokaryotic flagella is that it is composed of protein and those proteins are referred to as h antigens and these h antigens there are about 50 different types and they can be used for identification so that is one thing that we can use to identify different types of bacteria the h antigens in terms of function of the flagella the main function is locomotion and locomotion refers to getting the cell from one place to another and that's true for both both prokaryotic and eukaryotic flagella in terms of how they move prokaryotes move in a run and tumble type of way so it's called run and tumble because the prokaryotic cell will move in a straight line when it gets to a certain point it will tumble around sort of spin around and then it will run in a straight line and then it will sort of tumble and turn around and run again so it looks very unorganized if you can see it under the microscope eukaryotes on the other hand they seem to be more directed they don't do that run and tumble type of motion now motion the type of motion you can have you can have chemo taxes or photo taxes so chemo taxes taxes is movement chemo is in response to a chemical when we say chemical like chemo we mean a molecule phototaxis is movement in terms of light in relation to light so chemotaxis and phototaxis now both of these types of movements you can have positive and negative so positive chemotaxis means that the cell is moving towards the molecule negative chemotaxis means that the cell is moving away from the molecule phototaxis positive phototaxis means that the cell is moving into the light negative phototaxis means that it is moving away so an autotroph is the type of organism that needs light to perform photosynthesis so an autotroph would probably express positive phototaxes flagella can be used to help with identification in two ways first the h antigen which i mentioned before the type of protein that makes up the flagella and there are 50 different types the other way the flagella can aid in identification is by the arrangement how the flagella are arranged on the bacterial cell and for this there are four different types of arrangement peritricus monotrichus lophotricus and amphitricus and if you know the terms the prefix then it makes them very descriptive so we'll start out with monotriches first mono means one so this is where the bacterial cell has one polar flagella so that's monotrichus peri means surrounding like the pericardium is the membrane that surrounds the heart so peritricus means that the flagella are evenly distributed all over the bacterial cell then lophotricus lophotricus you may not know that term lofo means tuft so this means there's more than one flagella at one end there's a tuft of flagella the last one amphitrichus amphi means both and for this there's one flagella at both ends at each end so that is an amphitrichus and if you didn't figure it out trickis refers to flagella so peritricus spread all over the cell monotrichus one polar flagellum lofotrichus means the tuft so a tuft of several flagella at one polar end and amphitrichus means both so one flagella at each end and another term is a trickiest a means without so if a cell is called a tricus that means it has no flagella one special type of flagella is called an endo flagella or sometimes referred to as an axial filament and this is only found on spirochetes on those flexible spiral bacteria and in studying instead of having a flagella that extends out like a tail this flagellum is wrapped around the cell so it's wrapped around the cell and it's held there by an outer sheath now the purpose of this flagella is the same because when it rotates it creates a corkscrew motion and so the cell sort of corkscrews through fluids so the purpose is still locomotion but instead of extending out from the end of the cell it's wrapped around the cell the second type of appendage that i want to talk about are the fimbriae and frembriae are usually found on gram-negative bacteria mainly and if a bacterium has it it can have a few or up to a hundred of the fimbriae and these are usually evenly distributed over the surface of the cell and they are shorter than flagella and they are more rigid sort of like bristles along the cell and their main purpose is attachment so if a bacterial cell has frembriae that is but then it is better able to adhere to the tissues and it's more difficult for the bacterium to be flushed out of the body so for instance e coli e coli is normally found in the colon but sometimes the e coli can migrate up the urethra but every time the person urinates they can flush the bacteria out unless that bacteria has lots of fimbriae and it's able to adhere to the epithelial tissue and the urethra and then it's much more difficult to flush it out so having fimbriae will make a bacteria much more likely to cause disease it makes it more pathogenic the last type of appendage that a bacterium may have is a pilus pili for pleural pilus for singular and if a bacterium has a pilus it's only going to have one or two very few and they are longer than fimbriae usually and these are usually found on gram negative bacteria now a pilus can be used for motility but the main purpose of a pilus is for conjugation which is a type of sexual reproduction in bacteria conjugation is sometimes referred to as sexual reproduction bacteria but it's not the type of sexual reproduction that involves gametes and fertilization so it's not a true sexual reproduction but the process of conjugation involves dna transferring from one bacterium to another and to do this you start with an f positive cell f stands for fertility factor so it's when you have an f positive cell that has an f factor otherwise known as a plasmid and that f positive cell will form a pilus apillus to an f minus cell and then a copy of that plasmid is going to cross the pilus and go into the recipient cell converting that recipient cell into an f positive cell so the pilus is used to transfer a copy of the plasmid from one cell to another the dna that is transferred via a pillis to another bacterium is called a plasmid and a plasmid is a small circular piece of dna very small and on that plasmid often you find genes for toxins and for antibiotic resistance so if a bacterium can produce a pilus then that means it can transfer plasmids that can convey toxin production and antibiotic resistance to neighboring bacteria so being able to produce a pilus will make a bacterium more pathogenic because it has a more likely to produce a toxin and or be antibiotic resistant so far i've gone through the appendages the flagella the fimbriae and the pillows and i've also talked about the plasmid a small circular piece of dna so the next thing i want to do is talk about the layers of the cell the outermost layer of a prokaryotic cell of a bacterial cell is called the glycocalyx and glyco refers to sugar and calyx means coat so this is a sugar coat and this is mainly composed of carbohydrates uh glycoproteins glycolipids and remember glyco refers to sugar the main purpose of a glycocalyx is adherence it is sticky helps the cell to stick to surfaces to stick to rocks to stick to tissues also the glycocalyx is protective it can protect the cell from from different types of things there are two types of glycocalyxes that you can find on bacteria the first one is called a slime layer and the slime layer is often found on bacteria you find out in the environment in the soil in water on rocks and a slime layer is very unorganized in terms of the molecules that compose the slime layer and it's very loosely held to the cell and relatively thin and the purpose of the slime layer is mainly attachment so holding those bacteria to the substance it's on to the rock to the twigs and also another function of the slime layer is to protect from dehydration the other type of glycocalyx is the capsule and a capsule is the type of glycocalyx that you find on pathogens potential pathogens and when you look at the structure of the capsule it's much more highly organized in terms of the molecules it's very tightly held to the cell and it is relatively thick and of course part of the purpose of the capsule is attachment which will increase pathogenicity just like the fimbriae so if the bacterial cells are better able to adhere to the tissues then they're more difficult to flush out another benefit of the capsule is that it protects the bacterial cells from phagocytosis and phagocytosis phago is eat cytosis is cells it's when a cell tries to eat the bacteria and in the human body the white blood cells some of those will try to phagocytose the bacterium so if the bacterium has a capsule it will be protected from phagocytosis and this of course makes the bacteria more pathogenic because it's harder for your white blood cells to kill it related to the glycocalyx is an eps and eps is an extracellular polymeric substance so this relates to biofilms and biofilms are communities of bacteria that are growing together and what happens when you have these different bacteria growing together is that their glycocalyces sort of fuse together to form a large structure and this is the biofilm and again the main purpose of the glycocalyx is to adhere the bacteria or in this case the eps is adhering the colony of different species of bacteria to a surface and biofilms are very difficult to get rid of they're very strongly held to surfaces and in the human body the most common biofilm is on the teeth the plaque on the teeth so in your mouth you have different colonies of bacteria different types of bacteria they're growing together and their glycocalycies are sort of fusing together to hold those bacteria onto the teeth and that's the plaque that's on the teeth the outermost layer of the prokaryotic cell is the glycocalyx and as you move into the cell you will come across the cell wall and the main purpose of the cell wall is to protect the bacterium from osmotic lysis so bacterial cells tend to be hypertonic compared to the human body so when a bacterium is in the human body the direction of osmosis is into the bacterial cell so it's gaining water so it is in danger of rupturing but having a strong rigid cell wall prevents that osmotic lysis another purpose of the cell wall is for shape it helps hold the shape of the bacterial cell whether it's a coccus bacillus or spiral and for bacterial cell walls i mentioned a difference between bacteria and archaea i mentioned that bacteria have peptidoglycan that is the main component of a bacterial cell wall and if you remember our ke do not have peptidoglycan and there are two major types of cell walls gram-positive cell walls and gram-negative cell walls and so the type of cell wall a bacterium has can help with identification a gram positive cell wall is referred to as gram positive because dr graham figured out a staining procedure by which we can tell the differences between the two types of cell walls and this is a diagram of a gram-positive cell wall and here is the plasma membrane for orientation this is the plasma membrane and exterior to the plasma membrane is the cell wall and a gram-positive cell wall has many many layers of peptidoglycan so it is a very thick rigid barrier exterior to the plasma membrane a gram negative cell wall has a very different structure so here again is the plasma membrane interior to the cell wall and now when you look at the cell wall you notice two things first there's only one layer of peptidoglycan so the layer of peptidoglycan is very very thin the second thing you will notice is that exterior to the peptidoglycan is an outer membrane and the outer membrane is also has the basic structure of a membrane it's a phospholipid bilayer and has the same basic purpose as a membrane it is a selectively permeable barrier so this is a selectively permeable barrier so it can prevent different molecules from entering the cell it can prevent things like detergents salts enzymes and antibiotics from entering the cell so gram-negative cells because they have this outer membrane they tend to be more resistant to different chemical disinfectants and antibiotics compared to gram positive cells another characteristic that relates to the outer membrane is the presence of this molecule called a lipopolysaccharide so this is the lipopolysaccharide and the name is long but it tells you exactly what it's composed of it's composed of a lipid a lipo component and a carbohydrate a polysaccharide and these lipopolysaccharides are embedded in the outer membrane now the important thing about the lps the lipopolysaccharide is when the gram negative cell dies the outer membrane will rupture and it will release the lipopolysaccharide and the lipopolysaccharide will break into two essential parts the lipid a and the o polysaccharide now o polysaccharide there are over a hundred different types of o polysaccharide and this can be used for identification so that's another way to help identify cells the lipid a on the other hand is an endotoxin so it's an endotoxin and if enough lipid a is released that can induce shock in the patient so because of the lipopolysaccharide and this lipid a that is part of the lipopolysaccharide gram negative cells tend to be more pathogenic than gram-positive cells while the vast majority of bacteria have either gram-positive or gram-negative cell wall structure there is one group of bacteria we will talk about that has a different structure and they have acid fast cell walls and acid fast is another staining technique now the bacteria that have these types of cell walls are mycobacterium leprey which causes leprosy and mycobacterium tuberculosis so those are the two types of bacteria we will talk be talking about and they have acid fast cell walls and the difference with the acid fast cell wall is that there is very little peptidoglycan in the cell wall so very little peptidoglycan and instead they have many lipids and all of these are part of a lipid molecule and they also contain mycolic acid and that name comes from the mycobacterium and so these cell walls have very little peptidoglycan and they have mainly these lipids and waxes and those types of cell walls help protect the bacterium because it makes it very difficult for phagocytic cells to phagocytose these types of bacteria i have gone over the glycocalyx then moved into the cell wall and now the last layer of the cell is the cell membrane you should be very familiar with the structure of the cell membrane so the cell membrane is made of a phospholipid bilayer so two layers of these phospholipids you should know every phospholipid has a polar head or a hydrophilic head and two non-polar or hydrophobic tails when you mix them in water you automatically get a bilayer forming and you should know that the phospholipid bilayer is the main matrix for the membrane and this helps serve the major purpose of the membrane to be a selectively permeable barrier to control what goes into the cell and what goes out of the cell and in general nutrients are taken in and waste products go out so the cell membrane has the same function in prokaryotic and eukaryotic membranes now difference in the cell membrane of a prokaryotic cell in a prokaryotic cell there are many proteins that are bound inside the cell membrane and a lot of these proteins are enzymes in a eukaryotic cell membrane there are not very many enzymes bound to the cell membrane but in prokaryotes there are many enzymes in the cell membrane and another difference between prokaryotic and eukaryotic cell membranes is that eukaryotes have cholesterol embedded in the membrane and prokaryotes do not all right so i've gone through the appendages the flagella the firm brie the um pillars and i have gone through the capsule i type of glycocalyx the cell wall and the plasma membrane so the last thing i want to go through in the cell is what you find inside the cell so interior to the plasma membrane inside the cell you find the cytoplasm and technically the cytoplasm includes everything you find inside the cell when we're referring to the liquid part of the cell we call that the cytosol so that's the liquid part inside the cell and that is mainly composed of water and all sorts of dissolved nutrients waste products gases and so on the difference between the cytosol in a prokaryotic cell in a eukaryotic cell is that in a prokaryotic cell it is about 80 percent water and that's important to remember because in a eukaryotic cell it's usually about 90 percent water so by knowing the percentage of water you can figure out which cell is hypotonic which is hypertonic and you can figure out the direction of osmosis every prokaryotic cell must have a chromosome and the chromosome contains the essential genes that determine which proteins will be produced and those proteins determine the characteristics of that particular cell and the characteristics of a prokaryotic chromosome is that there is only one per cell it is circular and it is composed of dna only that is different than eukaryotic chromosomes which there are several there are many chromosomes they are linear and they are composed of dna complexed with a protein called histone in the prokaryotic cell the cell does not have organelles that is one of the definitions of prokaryotic cells but often in the cell you can see this area inside the cell and that is called the nucleoid and the nucleoid is the area where you find the chromosome so that's where you find the chromosome and the chromosome again is a circular piece of dna that contains the essential genes for the cell determines all the characteristics of the cell in addition to a chromosome all prokaryotic cells will have ribosomes and the purpose of the ribosomes are to produce proteins they perform translation which takes the mrna and creates the corresponding protein and the vast majority of those proteins will be enzymes so prokaryotic ribosomes and eukaryotic ribosomes serve the same function the difference is is in their size prokaryotic ribosomes measure 70s and that's a measurement that takes into account size and density eukaryotic ribosomes measure edf so the important point of that is you can tell the difference between eukaryotic and prokaryotic ribosomes they are slightly different some prokaryotic cells may have a plasmid and i've mentioned the plasmids before the only way a prokaryotic cell can get a plasmid is through conjugation so conjugating with an f positive cell and a copy of the plasmid is transmitted across the pillars and again the plasmids they contain genes for toxin production and antibiotic resistance so having a plasmid makes a cell more likely to be pathogenic and bacterial cells can have anywhere from zero plasmids if they've never conjugated to hundreds of plasmids depending on how often they have conjugated the last thing a prokaryotic cell may have inside are inclusions and inclusions can be identified as these darker staining areas in a cell and an inclusion is basically a storage area in the cell and there are all sorts of different inclusions some can be composed of polysaccharides or lipids or they can be areas where the cell is storing sulfur phosphate iron and it really depends on the type of cell but an inclusion is a storage area so that is all for the basic structure of a prokaryotic cell bacterial cell i've gone through the three types of appendages the flagella the fimbriae the pilus the three layers the capsule the cell wall the plasma membrane and then inside the cell all bacterial cell will have a chromosome one single circular chromosome composed of dna only they will have many ribosomes and they may have a plasmid depending on how often they have conjugated and they may have inclusions the last thing i want to talk about are endospores and endospores are specialized resting cells that are produced by only two types of bacteria two genus is a bacteria the first genus is bacillus and bacillus the species name is bacillus anthracis so it causes anthrax the other genus is clostridium and clostridium has several species clustering to tiny causes tetanus clostridium botulinum causes botulism and clostridium peripherals causes gaseous gangrene in a clostridium we might have heard before clostridium difficile more commonly known as c diff so these are the two genus names of bacteria that produce endospores endospores are produced by a process called sporulation and it is triggered when the environment is hostile like when there isn't there aren't enough nutrients there isn't enough water there are toxins in the environment something is wrong with the environment and when that happens that triggers the cell to copy its genome so it originally has one chromosome and it makes a copy of its chromosome and it moves the copy of the chromosome to the end of the cell then what it does it starts to build a cell wall a very thick strong tough cell wall around that copy of the chromosome and there's very little water in that structure and this is the endospore it's formed inside the cell once the endospore is mature then the cell ruptures and releases the endospore and that endospore is now highly resistant to all forms of disinfection it's highly resistant to heat to radiation to chemicals and antibiotics and the cell can remain in endospore form for many years i think in your book it talks about a an endospore that they germinated after about 250 million years or 25 million years old so these endospores are highly resistant to disinfection and sterilization and they can last for a very long time a living cell usually a living cell can only survive for a couple of minutes in boiling water and endospore can last for hours in boiling water so being able to produce an endospore makes a bacterium more pathogenic because it will be more difficult to kill and this is a photograph of bacillus and thoracis with an endospore stain and the cells you can see they are pink so this is a bacillus long thin cells and the endospores are green and you can see that they are forming inside the cells and some have been released already so that's an example of bacillus and thoracis that is going through sporulation this is the end of the lecture for prokaryotic cells so this lecture i went through the different parts of the prokaryotic cell different structures and one of the things i told you to keep in mind is how they compare to eukaryotes and throughout the lecture i mentioned some ways that they compare when i go through the eukaryotic cell lecture i will go through those things again and also remember go through the different structures think about how they're used for identification or how they can be used for identification and also how the structures relate to the pathogenicity of the bacteria