Question

The following table outline the unknown gram-positive bacterium that was isolated using these biochemical test:

Catalase Test (positive/negative)	Coagulase test (positive/negative)	Oxidase test (positive/negative)
Positive	Negative	Positive

1) Briefly describe which gram-positive bacterium it was and why was it?

2) In the mixed culture, you isolated a Gram-positive bacterium. Describe the significance of its isolation from a clinical sample from:

a) A normally sterile site (e.g. blood, joint aspirate)     […../5]

b) A non-sterile site (e.g. wound swab, throat swab) [……/5]   

Answer 1

1) The organism is Staphylococcus sciuri

i. Catalase test is done to distinguish Staphylococcus (produces catalase and thus gives positive result with the test) between Streptococci ( do not produce catalase and thus gives negative result with the test).

The test involves treating a day old bacterial culture with Hydroxygen peroxide (3%), if catalase is present, Hydrogen peroxide is cleaved into water and oxygen, which is observed as production of bubbles. This inference is taken as postive result. Since staphylococcus is having catalase, it answers the test

ii. Coagulase test: Used to differentiate Staphylococcus species which are having coagulase or clumping factor, from species which are devoid of it. Agglutination indicates the positive result. Since Staphylococcus sciuri, does not have coagulase, it gives negative result to this test.

iii. Oxidase test: This test is used to detect the presence of cytochrome c oxidase in the organism, if present it reacts with tetramethyl para phenylenediamine, whose oxidation will result in purple color. This is taken as positive inference. As Staphylococcus sciuri is having cytochrome c oxidase, it gives positive result.

The scheme of tests that helps in drawing conclusions for identification of different gram positive bacteria is described in the following figure:

2) a) A normally sterile site:

Findings in Blood: Gram-positive cocci like staphylococci, streptococci, diplococci like streptococcus pneumonia , Gram positive bacilli like clostridium, Listeria or diphtheriods

Findings in Joint aspirate: Gram-positive cocci like staphylococci, streptococci

2) b) A non-sterile site:

Findings in wound swab: Gram-positive cocci like staphylococci, streptococci, Gram positive bacilli like clostridium, Listeria or diphtheriods

Findings in throat swab: Gram-positive cocci like staphylococci, streptococci, gram positive oval forms with hyphae(yeasts)

Significance of the findings: Staphylococcal infections may lead to boils, food poisoning, impetigo, toxic shock.

Streptococci infections may represent sore throat, fever, low blood pressure, tissue damage, infections to the blood and other major organs

Lactobacilli and other diphtheriods in throat swab are not clinically significant in most of the cases

Clostridium species signify the presence of food poisoning, tetanus, Listeria presence denotes listeria infection with symptoms like flu, diphtheroids are common findings in throat, mouth and pharynx