so this is the second half of a lecture on selective and differential media for the week of April 5th 2029 you talked about in part one what selective and differential media are and so I just wanted to remind you that they're mainly used to identify bacteria based on the biochemical properties of different groups and differential and selective media can be either one or the others so they can be just selective just differential or they can be both and all the examples in this particular lecture are selective and differential those are the first type of media that I'm going to talk about in this lecture is EMV or Aon methylene blue auger you can see it has this kind of nice dark bluish purple color when it's uh Nanaki elated and ambi is selective and inhibits the growth of gram positive bacteria selecting for gram negatives and EMV is also differential and the main thing that EMB can differentiate between our microbes that can ferment lactose or lack positive microorganisms and microbes that can't or like negatives and you can see here lactose negative microorganism growing on the far left a lactose positive growing in the middle and then another lactose positive growing on the far right the way that EMB differentiates between Lac positive and Lac negative or lactose fermenters and non lactose fermenters is basically by the color of the colonies and so strong lactose fermenters or a strong Lac positive organisms will turn purple as you can see up here even all the way up into a dark black when grown on ambi microbes that can sort of or weakly ferment lactose will appear as a pink color as you can see down here on the right and non lactose fermenters will have no color at all as you can see up here in the top right and so EMV is selective for gram-negative bacteria and then differential for lactose fermentation another type of selective and differential media is macconkey agar and so you can see an undocumented plate up here it kind of has this light pinkish purple color and McConkey contains bile salts and crystal violet that actually inhibit the growth of gram positive organisms and select for gram negatives and so you won't see any gram positives growing McConkey talker McConkey can also differentiate once again between lactose fermenters or lac positive organisms and lactose non fermenters and the way that McConkey does this is using a pH indicator called neutral red in the sugar called lactose and so by incorporating lactose and neutral red into the media you give a lactose fermenter the potential to ferment lactose when that happens the pH of the media will drop because it produces lactic acid and neutral red will turn pink or dark kind of this dark pink red here on the left when lactose is being fermented the pH of the media is turning low and so a Lac positive organism will McConkey will have this red color whereas a lac negative is unable to ferment lactose the pH of the media will go up and increase pH and neutral red will show up as a yellow media and so you can see a lot of positive on the left laughs negative on the right like positive peed pink OMA conky's and Lac negative being yellow and just reminder that all of the organisms that grow out macaques will be grabbed negative because it's selected against grant positives the next type of media that's both selective and differential and that I talked about is mannitol salt agar or MSA and unlike McConkey and ambi MSA inhibits the growth of gram-negative bacteria and selects for gram positives and it does this by containing a very high amount of salt or NaCl so seven point five to ten percent which is a pretty high concentration of salt will inhibit growth of gram negatives and not gram positives so that thick peptidoglycan layer can provide some protection for the gram positives against this high salt which is why they can grow fine on a plate like you can see I'm here on the top rate so this is an undocumented plate and once you inoculate MSA plate with your organism it can be used to differentiate between organisms that ferment mannitol which is another type of sugar and those that don't right and so if you put a gram-negative bacterium on MSA you won't see any growth because it selects against that but gram positives will grow and then the addition of this mannitol sugar as well as a pH indicator called phenol red can help you differentiate between whether a particular bacteria can ferment mannitol or not and so phenol red turns yellow at a low pH and when mannitol is being fermented it produces an acid that lowers the pH of the medium and therefore turns the whole thing yellow and so a mannitol positive bacteria or Mantell fermenter which are yellow and MSA and the opposite is true from mantle nog fermenter and so you can see on this on the right here a mannitol negative result where this bacterium cannot format mannitol the page of the media will actually increase and you know red will turn a deeper darker pink color and so mannitol positive on MSA is yellow and mannitol negative is kind of a dark pink or red another type of selective and differential media is bile F colon auger or B ei which is selective against gram-positive bacteria it contains a high amount of bile salts that allow gram negatives to grow really well and inhibits growth of gram positives and an undocumented plaintiff can be seen up here and it has kind of a typical like beige-ish color but EEA can be used to differentiate between two specific types of bacteria the streptococcus bacteria and Enterococcus bacteria because enterococci are able to hydrolyze s : and s : is incorporated into this media along with another compound called ferric citrate and the hydrolysis project of s : reacts with ferric citrate to turn the medium black and so you can use a ve a plate and look for color change from beige to black to differentiate between an Enterococcus which would have a black or dark brown media and a streptococcus which will remain beige because it's unable to hydrolyze escalade and therefore unable to turn the media black HEC to nagger is another selective and differential media that has a really pretty color kind of a gushers blue if you will and heck to it is selective with the addition of all these dyes as well as salt against most gram positive bacteria so gram negatives will grow fine on HEC to an auger whereas gram positives won't and so the onion acylated plate can be seen up here it's got this nice deep teal and once you play organisms on it you can use Hector an auger to differentiate between lactose fermenters Lando's not firm enters a black positive and lack negative bacteria and so a lot negative bacteria that can't ferment lactose won't change the color of the media at all whereas a lactose fermenter will change the color of the media from deep blue to orange there is a pH indicator called acid fusion in heck twin media and acid fusion turns yellow to salmon and a low pH so when lactose is being fermented the media is getting more acidic the page is getting lower and it turns this orange color and so you're lac positives will look orange whereas like negative bacteria will remain the media will remain blue and hack tone is interesting because it can also differentiate between the bacteria that can produce hydrogen sulfide and ones that can't and so in addition to these dyes as well as bile salts heck twin contains sodium thiosulfate and ferric citrate and these compounds can interact with hydrogen sulfate and produce a black precipitate in the same way that TSI agar time and so what you'll notice is that when a bacterium can produce hydrogen sulfide the colonies themselves are filled with this black precipitate and that's how you distinguish between a hydrogen sulfide positive bacteria which has black colonies and one that is hydrogen sulfide negative which would have like greenish of bluish colonies and so Hector is interesting because I can differentiate not only between different lactose fermenters but also different hydrogen sulfide producers you