Microbiology Quick Reference

Agars

Agar	Enrichment	Selective	Differential Features	Use
Blood	5-10% sheep blood		Shows hemolysis	General growth medium
Chocolate	5-10% lysed sheep blood 2% hemoglobin + supplements			Growth of fastidious organisms
CNA	5-10% sheep blood	Gram positives grow Colistin Naladixic acid	Shows hemolysis	Growth of gram positives and yeast
MacConkey		Gram negatives grow Bile salts Crystal violet	Shows lactose fermentation	Grow of gram negatives

Agar	Features	Use
Meuller Hinton
MH + Sheep's blood	Enrichment: sheep blood	AST for specific organisms Streptococcus pneumoniae
Thioglycollate broth	Enrichment: hemin & vitamin K Differential: O2 use based on location of growth in tube	General growth medium, allows presumption of atmospheric growth conditions
Enrichment Media	Usually broth Grows fastidious or a specific organism from mixed specimens Contain differential reagents to suppress normal flora for a short period of time Requires subculture within ~ 12-18h	Selenite broth Tetrathionate broth Brain-heart Infusion broth Cooked meat broth
Sorbitol MacConkey	Selective: for gram negatives Differential: sorbitol fermentation	Enterohemorrhagic E. coli (non-sorbitol fermenters) vs other E. coli (sorbitol fermenters)
Salmonella-Shigella	Selective: bile salts, sodium citrate, brilliant green Selects for gram negative enteric pathogens Differential: lactose fermentation + H2S production	Isolate enteric pathogens
Hektoen Agar	Selective: bile salts Selects for gram negative, mostly enteric pathogens Differential: lactose, sucrose, and salicin fermentation + H2S production	Isolate enteric pathogens
Cefsulodin Irgasan Novobiocin (CIN)	Selective: cefsulodin, irgasan, novobiocin, sodium desoxycholate, crystal violet Inhibit gram + and normal stool flora Differential: mannitol fermentation	Isolate Yersinia enterocolitica
Campylobacter agar	Enrichment: 10% sheep blood Selective: vancomycin (inhibit gram +) amphotericin B + polymyxin B (inhibit yeast/fungus) trimethoprim (prevent Proteus swarming) cefoperazone (inhibit gram -) Sodium bisulfite (creates microaerophillic environment)	Isolate Campylobacter
Mannitol salt agar	Selective: 7.5% salt Selects for halophiles Differential: mannitol fermentation	Screen for Staphylococcus aureus
New York City agar	Enrichment: lysed blood, yeast dialysate, and plasma Selective: vancomycin, colistin, amphotericin B, and trimethoprim

Stains

• Gram stain
• Ziehl-Neelsen (acid fast)
  • Uses heat for uptake of carbolfuchsin
• Kinyoun (acid fast)
  • Higher phenol concentration for uptake of carbolfuchsin
• Auramine/Auramine-Rhodamine (acid fast)
  • fluorescent dye binds to mycolic acids
• Acridine Orange (organisms without cell wall)
  • fluorescent dye binds to nucleic acid in cells
  • useful for organisms without cell wall (e.g., Mycoplasma)
• Calcofluor white (fungi)
  • Fluorescent dye binds to cellulose and chitin

Kinyoun = cold, Acridine = nucleic acids, Calcofluor = chitin

Test	Positive QC	Negative QC	Uses
Catalase	Staphylococcus spp.
Coagulase	S. aureus	Other Staphylococcus
Staphaureux (Latex agglutination)
PYR	S. lugdunensis

Staphylococcus species

Staphylococcus aureus

• GPC clusters
• Catalase +
• Coagulase +
• Staphaureux +

If coagulase/Staphaureux negative, then CoNS:

• S. lugdunensis: PYR +
• S. saprophyticus (females 12-60): Novobiocin resistant
• Others: S. epidermidis, other CoNS

Streptococcus species

• GPC pairs & chains
• Catalase -

Lancefield grouping mainly used for ID

Lancefield Grouping	Organism	Features	Clinical Relevance
Group A	S. pyogenes	Beta-heme PYR +	iGAS
Group B	S. agalactiae	Beta-heme
Group D	S. bovis
	Enterococcus
Group C	Streptococcus dysgalactiae subsp equisimilis	Beta-heme	Normally commensal
Group G
	S. pneumoniae	Alpha-heme Bile soluble Optochin sensitive (≥14mm)
	Viridans Streptococci

Catalase

Oxidase

PYR