Microbiology Lab Test 2

major Staphylococcus species

Staphyloccocus aureus, Staphylococcus epidermidis

major Streptococcus species

Streptococcus pyogenes, Enterococcus faecalis

characteristics of Staphylococcus aureus

gram-positive cocci in clusters that can cause many kinds of human infections, though it can also be part of the normal biota; can grow on CNA & MSA, positive catalase test, ferments mannitol, positive beta-hemolysis, positive coagulase test, positive DNAse test

characteristics of Staphylococcus epidermidis

gram-positive cocci in clusters that is part of the normal biota on skin and mucous membranes, though can cause opportunistic infections; can grow on CNA & MSA, positive catalase test, cannot ferment mannitol, negative hemolysis, negative coagulase test, negative DNAse test

characteristics of Streptococcus pyogenes

gram-positive cocci in chains, common pathogen that causes strep throat; can grow on CNA but not MSA, negative catalase test, cannot ferment mannitol, positive beta-hemolysis, negative bile esculin test, susceptible to bacitracin

characteristics of Enterococcus faecalis

gram-positive cocci in chains that is part of the normal biota in the human GI tract, but can cause opportunistic infections; can grow on CNA but typically has pseudogrowth on MSA, cannot ferment mannitol but may cause a pseudo positive, negative catalse test, negative hemolysis, positive bile esculin test, resistant to bacitracin

CNA (colistin-nalidixic acid) agar

selective and differential medium that inhibits growth of gram-negative bacteria, therefore all growth is gram-positive

catalase test

uses H2O2 to test for the ability to break it down into water and oxygen; positive test is vigorous bubbling, indicating the presence of catalase; useful for differentiating staph. from strep. organisms

MSA (mannitol salt) agar

selective and differential medium used to isolate and differentiate Staphylococcus species (salt inhibits all other from growing); all growth is positive and presumed to be Staphylococcus; fermentation may occur, indicated by the agar changing from pink to yellow (S. aureus does this)

BAP (blood agar plate) agar

differential medium containing trypticase, soybean extract, and animal blood; most organisms can grow on it, but some are able to produce hemolysins that cause RBCs to lyse, indicated by color change in the agar; positive alpha-hemolysis=green agar , positive beta-hemolysis=clear agar, positive gamma-hemolysis (aka non-hemolytic)=red agar (no change)

coagulase test

uses rabbit plasma to test for the presence of coagulase; positive test indicated by clumping; used to differentiate Staphylococcus species since S. aureus produces coagulase as a virulence factor

DNAse agar

differential medium containing DNA and methyl green to test an organism’s ability to produce DNAse; organisms producing DNAse are able to digest the DNA in the agar, changing the color from green to clear; S. aureus & S. marsecens are positive for the DNAse test

BE (bile esculin) agar

selective and differential medium containing esculin & sodium azide to test an organism’s ability to hydrolyze esculin; positive test indicated by dark brown or black agar; often used to identify E. faecalis, which can hydrolyze esculin

bacitracin sensitivity test

uses the antibiotic bacitracin atop a streak plate to identify Group A Streptococci (such as S. pyogenes) which are susceptible to the antibiotic, displaying a zone of inhibition around the disc

strep throat tests

rapid strep test is accurate most of the time but not all, therefore a culture on BAP is always needed in addition, and a second culture is performed on BAP with bacitracin if the first culture has numerous beta-hemolytic colonies

indole test

performed on TSB or SIM agar, tests for the organism’s ability to break down tryptophan into indole using Ehrlich’s reagent; positive test indicated by a red/pink ring

methyl red test

performed on MRVP broth to test for the organism’s ability to break down glucose into acidic produce using methyl red as a reagent; positive test indicated by a bright red broth

citrate test

agar slant that tests for an organism’s ability to use citrate as a nutrient; positive test indicated by dark blue colored agar as the pH becomes more alkaline

urease test

agar slant that tests an organism’s ability to break down urea into ammonia and carbon dioxide using phenol red as an indicator; positive test indicated by hot pink agar

PAD test

agar slant that tests an organism’s ability to break down phenylalanine; reagent ferric chloride reacts with the enzyme PAD to turn the surface of the agar dark green

triple suger fermentation test

TSI agar slant tests an organism’s ability to ferment lactose, sucrose, & glucose using the indicator phenol red; positive test is indicated by yellow agar (slant=lactose & sucrose, butt=glucose)

gas production test

TSI agar slants test an organism’s ability to produce gas; positive test indicated by bubbles, cracking, or popping up of the agar

H2S production test

TSI or SIM agar slants test an organism’s ability to produce hydrogen sulfide using cysteine and FeSO4 as indicators; positive test indicated by black agar

motility test

SIM agar tube test an organism’s motility with a needle puncture in the agar; positive test indicated by growth throughout the agar tube or general cloudiness of the agar

voges-proskauer test

performed on MRVP broth to test for an organism’s ability to break down glucose into acetoine using o’meara’s reagent; positive test indicated by a pale pink band at the top of the broth

enterobacteriaceae

family of bacteria that are normal inhabitants of the intestinal tract including coliforms such as E. coli as well as non-lactose fermenters

enterobacteriaceae coliforms

gram-negative, non-spore forming, lactose fermenting bacilli including species from the genera Escherichia, Enterobacter, Edwardsiella, Serratia, Arizona, and Citrobacter, all of which are cnsidered normal microbiota of the GI tract but can cause disease in other areas

non-lactose fermenting enterobacteriaceae

gram-negative bacilli that cannot ferment lactose; includes some species that are normal microbiota of the GI tract such as Proteus, Providencia, and Morganella, as well as pathogens such as Salmonella, Shigella, and Yersinia

tests performed on SIM agar deep

H2S production, idole production, motility

tests performed on TSI agar slant

triple sugar fermentation, gas production, H2S production

tests performed on MRVP broth

methyl red, voges-proskauer

macconkey (MAC) agar

selective and differential medium used for isolating and differentiating enteric bacteria; selective agents are crystal violet dye and bile salts, differential agent is lactose, pH indicator is neutral red; agar turns hot pink/red in the presence of acid, lactose-fermenter colonies are dark pink/red (E. coli will produce a halo) and non-lactose fermenter colonies are colorless

eosin-methylene blue (EMB) agar

selective and differential medium used for isolating and differentiating enteric bacteria; selective agents are eosin dye and methylene blue dye, differential agent is lactose; lactose-fermenting colonies are pink/red/purple/ or metallic (E. coli) and non-lactose fermenting colonies are colorless

mueller-hinton (MH) agar

medium containing nutrients including beef extract and starch; used for kirby-bauer method for testing antibiotic sensitivity