Streptococcus - Ex. 50

Gram +, small, round

tend to grow in pairs or chains

catalase negative

1) put in groups by lancefield based on their similar antigens recognized by immune system --> Lancefield groups, named A through O

2) also put into four groups: pyogenic (pus-forming), viridans, enterococcus, lactic

3) also can be categorized based on ability to do hemolysis (destroy RBCs)


TESTS:
1) BAP
2) bacitracin A sensitivity
3) optochin sensitivity
4) bile esculin test
5) salt tolerance test

Blood agar plate

beta hemolysis - complete clearing
alpha hemolysis - partial clearing, with green-brown halo around colonies
gamma - growth but no hemolysis

Disk of bacitracin A, antibodiotic, added to quadrant 0 of BAP to test for zone of inhibition - indicative of sensitivity to drug

group A - strep pyogenes - sensitive
group B - strep agalactiae - resistant

Chemical - disk placed on BAP quadrant 0 - look for zone of inhibition

sensitive - strep pneumoniae (pneumococci)
resistant - everything else

Todd Hewitt broth

tests for susceptibility to lysis in the presence of a bile salt (desoxycholate)

after growing in broth for 48 hours, add pH indicator & desoxycholate reagent to see whether there's a clearing of turbidity from the medium

clearing = + for bile solubility

used to differentiate between the bacteria that are alpha/gamma hemolysis

positive - strep pneumoniae

Selective for and used to differentiate between Group D & Viridans group

tests for ability to hydrolyze escule, a glycoside compound containing glucose

bile esculin slant tubes

esculin hydrolysis --> esculetin product --> esculetin reacts with ferric citrate in the media --> brown color on slant

media also has bile, which inhibits growth of most gram + orgs, except Group D & viridans streptococci - selective for these groups

6.5% NaCl tryptic soy broth

used to differentiate within Group D streptococci, between the streps & the enterococcus

most group D cannot grow at this salt [ ], but enterococcus can

Most medically important beta hemolysis species of strep is group A strep pyogenes - causes strep throat (pharyngitis) & some strains cause scarlet fever

bacitracin A sensitive
optochin resistant

Beta-hemolytic
causes small number of human diseases
bacitracin A resistant
optochin resistant

Pneumococci

grows as diplocci - two cocci cells growing together surrounded by a single capsule

causes lobar pneumonia; associated with ear infections & meningitis


differentiated by: alpha hemolytic, optochin sensitive, bile soluble

Consist of streps & enterococcus faecalis

can hydrolyze bile esculin, optochin resistant, bile insoluble, alpha/gamma hemolytic

enterococcus faecalis - salt tolerant
other group D strep - e.g. strep bovis - salt intolerant

enterococcus - normally found in large intestine; can cause UTIs

Strep mitis

optochin resistant, alpha/gamma hemolytic, bile insoluble, cannot hydrolyze esculin