Question

Being aware of your expertise in microbiology a colleague has sent you four different strains of unknown bacteria and asks you to help in identifying each isolate.

Table 1 lists some characteristics you have determined for each isolate (Strain A, Strain B, Strain C and Strain D).

Table 1. Characteristics of four unknown bacteria isolates.

Test	Strain A	Strain B	Strain C	Strain D
Gram stain	Gram positive	Gram negative	Gram positive	Gram positive
Cell morphology	Rod	Rod	Rod	Coccus, in clusters
motility	motile	motile	motile	Non-motile
Aerobic growth	yes	yes	no	yes
Anaerobic growth	yes fermentative	no	yes fermentative	yes fermentative
Pigment of colonies	white	cream fluorescent	white	white
Growth on MSA	No growth	No growth	No growth	Yes, growth
Endospores	yes	no	yes	no
Catalase test	positive	positive	negative	positive
Oxidase test	negative	positive	negative	negative

1. Based on the information in Table 1 suggest a possible genus for each strain. Clearly explain the key characteristics and your reasoning for your proposed identifications. It is NOT necessary to provide identification to species.                        

b. What techniques could you use to confirm your suggested identification for each isolate and enable you to determine identification to species?

Answer 1

A. starin A: strain is Bacullus. ait is rod shaped, Gram positive, motile, which is oxidase negative and catalase postive. It does not grow on MSA as the media has high salt concentration that can be tolerated by Staphylococcus.

strain B: it is definitely Pseudomonas. it is a Gram negative, oxidase postive and catalase postive bacillus which forms yellowish colonies.It cannot grow on MSA.

strain C: it is another Gram positive bacilli i.e. Clostridium. the characteristics of bacteria are white colony forming, catalase negative as well as oxidase negative bacteria. endospore is terminal in nature.

strain D: coccus shaped Gram positive bacteria called staphylococcus. the important point is it grows on MSA. it is select8ve for Staphylococcus only. it is calatase positive and oxidase negative.

B. for all the strains, species identification includes other various biochemical tests like urease, nitrates etc. other than that, various antibiotics and advance methods like PCR can be used. For example, Staphylococcus epidermis is sensitive to novobiocin. this can be used to differentiate different species of Staphylococcus. Pseudomosa aeruginosa is the only Gram negative bacteria that is positive for both oxidase and catalase. so, these methods can be used to identify various species