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If the microscope clearly shows bacteria of cocci morphology but it is difficult to determine if they are Staphylo- or Streptococcus, are catalase tests a good way to differentiate between the two since Staphylococcus are positive and Streptococcus are negative? Are there any traps/exceptions?

Thanks!

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Generally yes: streptococci are catalase negative while staphylococci are catalase positive.

The catalase test is important in distinguishing streptococci (catalase-negative) [from] staphylococci which are catalase positive.

Unlike Staphylococcus, all streptococci lack the enzyme catalase.

However, there are exceptions to every rule:

Catalase negative Staphylococcus aureus.

Catalase positive Streptococcus sp.

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