Microbiology Lab Diagnostic Flowchartsby Shishin Yamada

last updated: September 23, 1998

Gram Positive Lab Flowchart

Gram Negative Lab Flowchart

© "First Aid for the USMLE Step 1" by Appleton and Lange.

Bacterial Cocci*Many thanks to Jason Collins for citing reference material for these charts

(from "The Microbiology Companion" book by ALERT and ORIENTATED publishers)

Species Gram DiagnosticReactions

Diseases

Staphylococcus–aureus (coagulase +, beta)–epidermidis (coagulase –)–saprophyticus (coagulase –)

+pale yellowcolonies, cocciin grape-likeclusters

Catalase + AbcessesFood poisoningToxic shock syndromePneumonia

Streptococcus–pyogenes (group A, beta)–agalactiae (group B, beta)–bovis (group D, alpha)–pneumoniae (alpha)–viridans (alpha)

+off-white smallcolonies, cocciin grape-likeclusters

Catalase – Pharyngitis, rheumatic feverPyoderma, acute glomeronephritisPneumoniaMeningitisSubacute bacterial endocarditis

Enterococcus–enterococcus faecalis

+white mediumcolonies, cocciin long chains

Catalase – EndocarditisUTI

Neisseria–meningitidis (ferments maltose)–gonorrhoeae (cannot ferment maltose)

–small, kidney-bean shapeddiplococci

Oxidase + MenigitisUrethritisPID

*KEY:Catalase = Mix with 3% hydrogen peroxide. Catalase + if forms bubbles.Coaguase = Check for coagulase enzyme. Coagulase + if plasma clots, clumps or forms precipitate.Oxidase = Determines presence of cytochrome-c in bacteria. Oxidase + if paper turns dark purple quickly.

Alpha = alpha hemolysis (very small clear area of hemolysis around colonies on blood agar)Beta = beta hemolysis (large clear area of hemolysis around colonies on blood agar)Gamma = NO hemolysis (no signs of hemolysis around colonies on blood agar)

Fermentation of specific sugars means that they change the pH of the tube over 48 hours. Fermentation is + forwhen the upper third of the tube turns yellow, while the bottom two-thirds remains red.

Gram Positive Rods*Many thanks to Jason Collins for citing reference material for these charts

(from "The Microbiology Companion" book by ALERT and ORIENTATED publishers)

Species Spores Diagnostic Reactions DiseasesBacillus–anthracis (non-motile, catalase +)–cereus (motile!)

YES Only AEROBIC Gram +rod that forms spores.

Colonies grow in longchains.

Anthrax exotoxinFood poisoning

Clostridium–botulinun (blocks Ach at NM junction)–tetani (Inhibits GABA, glycine)–perfringens (food poisoning, necrosis)–difficile (inflammatory lesion of colon)

YES Only ANAEROBIC G+rod that forms spores.

When inhibitsneurotransmitters, often seeflaccid paralysis (Ach) orspastic paralysis (GABA,glycine)

BotulismFood poisoningWound infectionsGas gangrenePseudomembranous colitis

Corynebacterium–diptheriae

NO Selective medium:Loeffler's agar; sincedifficult to separate

Diptheria (URI).CDEF = CorynebacteriumDiptheriae blocks ElongationFactor; so inhibits protein synth inall eukaryotic cells

Listeria–monocytogenes

NO Catalase +Beta hemolyticTumble-weed motilityLPS-like componentIntracellular reproductionin mononuclear phagocytes

DOES NOT produce exotoxinsOpportunistic meningitis

ENTERIC Gram Negative Rods*Many thanks to Jason Collins for citing reference material for these charts

(from "The Microbiology Companion" book by ALERT and ORIENTATED publishers)

Species Oxidase Growth onMacConkey

GlucoseMetabolism

Diseases

Enterics–Escherichia coli (metallic-green on EMB)–Shigella (non-motile, no flagella)–Salmonella (S. typhi makes a little H2S, swims like a salmon in body for systemic infection)–Klebsiella pneumonia–Enterobacter aerogenes–Proteus vulgaris(H2S gas, "swarms")–Yersinia pestis

– + Fermentsglucose

Traveller's diarrhea (ETEC)Dysentery (EIEC)Diarrhea and vomiting (EPEC)Hemolytic colitis (EHEC)Very bloody diarrhea (Shigella)Enterocolitis/gastritis (Salmonella)Nosocomial/pneumonia (Kleb)UTI (Enterobacter, Proteus)Bubonic plague (Yersinia)Many other shitty effects!

Pseudomonoadaceae–Pseudomonas aeruginosa (smells like grapes, procyanin)

+ + Oxidizesglucose

Pneumonia, meningitisUTI, Burn infections

Vibrionaceae–Vibro cholera (comma-shaped)

+ + Fermentsglucose

Cholera ("rice-water" stools)Diarrhea

Campylobacteraceae–Campylobacter jejuni (curved or S-shaped)–Helicobacter pylori (curved or S-shaped)

+ + Inert toglucose

Diarrhea (C. jejuni)Gastritis (H. pylori)

RESPIRATORY Gram Negative Rods*Many thanks to Jason Collins for citing reference material for these charts

(from "The Microbiology Companion" book by ALERT and ORIENTATED publishers)

Species Lab findings DiseasesHaemophilus influenza Coccobacillus

Growth requires factor V (NAD) andfactor X (heme). Found in old fashioned(not modern) chocolate agar

URIPneumoniaMeningitis

Legionella pneumophila Stains very lightly Legionaire's diseasePneumonia with fever, headache,diarrhea, vomiting

Bordetella pertusis Selective medium: Bordet-Gengou agar Whooping cough (only humans)

ZOONOTIC (associated with an animal)Gram Negative Rods

Species Lab findings DiseasesFrancissella tularensis From rabbits, e.g. Lawnmover-vs-rabbit

nest case. Spread by ticks.Caseating granulomas can formthroughout body. Intracellulargrowth in monocytes and PMNsallow it to evade antibodies andcomplement-mediated lysis.