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Basic Bacterial Morphology  Bacteria and fungus (mold and yeast) grow in a variety of shapes, sizes and colors.  Although it is difficult to make a definite identification of cultures growing in broth or solid media many have distinguishing characteristics that give us clues to identification.  Many Microbiologist can make a tentative identifications by growth characteristics but definitive id requires further testing (ie Gram stain, chemical testing, etc).
Isolation of Colonies: Streak Plate
The first step in identifying an organism is to isolate the individual bacteria/yeast/fungus.  This is done  by doing a streak plate for isolation.




The basis of the streak plate is to dilute the sample on the agar plate by dividing the plate into either 3 or 4 quadrents with the initial streak being at full strength.  After the initial streak the loop is flamed before each streak which dilutes the sample down to individual colonies.









Examples of a 3 quadrant streak plate done to isolate colonies.  …
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Nitrate Reduction/Nitrate Reductase TestSome bacterial species are able to reduce nitrate (NO3)  to nitrite (NO2)  using the enzyme nitrate reductase in an anaerobic process by using molecules other then oxygen, nitrate, as a terminal electron acceptor.  To determine if a bacteria is able to reduce nitrate they are grown in a nitrate broth and a series of chemical are added to detect nitrate reduction.

Nitrate reduction test for the reduction of nitrate in nitrate media, to determine if the bacteria contains nitrate and/or nitrite reductase. Organism were incubated for 48 hr's at 37 degree's in nitrate media then reagents added.  The addition of these reagents test for the presence of nitrites and the media will turn red if nitrites are present.
Nitrate broths after the addition of 10 drops reagents A (sulfanilic acid in acetic acid) and B (nn-dimethyl-1-naphthylamine in acetic acid).  A) Alcaligenes faecalis showed no color change indicating that zinc must be added to comple…
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Streptococcus Identification

Identification of Streptococcus spp can cause a range of infections from throat infections, toxic shock an a number of other skin and organ infections that can be mild to deadly. So it is important to be able to identify and differentitate between the different Streptococcus spp for treatment.  After a sample has been isolate and a culture grown a Gram Stain is preformed. Gram stains are the first test performed and  Gram reaction and cell morphology dictates the next test performed.  Streptococcus spp are Gram (+) and have a rope/chain like appearance.  


S. pyogenes (causes Strep throat).  The coccuc/ovoid Gram (+) shaped cells form long distinctive chains ("strepto" greek for twisted chain).  Once the Gram stain is done and Gram (+) coccus are seen then the catalase test is done to differentiate between Staphylococcus and Streptococcus (see blog on Staphlyococcus Identification).  Since Streptlococcusspp are Catalase (-) you proceeded with the f…