Question

DNase test is to determine presence of DNase. DNase-testing agar medium contains dissolved-DNA and an indicator (Methyl green). However, the DNase-testing agar medium sometimes contains only dissolved-DNA (without indicator, methyl-green). In this case (without the indicator methyl green), after the overnight culture of the unknown bacterial species, 1 N HCl solution will be flooded on the cultured- cells. Then, the results are interpreted as a clear zone (positive-reaction) around the cellular growth or opaque (negative reaction) around the cellular growth. 1. In this experiment, three bacterial species (E. coli, Serratia marcescens, Staphylococcus aureus, Staphylococcus epidermis) were used. Among them which one will be the strongest DNase-positive? (Select only one). 2. Is DNase-positive bacterium considered as a human pathogen? Yes or No. Explain your answer.

Answer 1

1. The DNAse test is used to identify the deoxyribonuclease enzyme producing organisms. This is indicated by using the approach given in the question. Out of the four species, E.coli and staphylococcus epidermidis are DNAse negative and hence will give a negative result. Out of the two species remaining, staphylococcus aureus is a DNAse positive organism, but the 1N HCl used is bactericidal to this microbe and hence Serratia marcescence will giving the DNAse positive test. Hence the answer is: Serratia marcescence.

2. Most of the DNAse positive microbes are pathogens. For example staphylococci aureus is a pathogen and the serratia marcescence is an opportunistic pathogen. Hence the answer is yes.

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