Staphylococci and Other Gram-positive Bacteria Quiz

See Coagulase-negative Staphylococci and Enterococci. D fits enterococci. These round or slightly oval bacteria retain the purple Gram stain and commonly remain attached as pairs or short chains. Their usual catalase-negative reaction means that adding hydrogen peroxide does not produce the brisk oxygen bubbling expected with staphylococci. Enterococci normally inhabit the bowel, but can cause urinary, bloodstream and heart-valve infections when they reach other sites. Shape and catalase narrow the possibilities; further identification is needed because streptococci share several of these features.

Staphylococci are the catalase-positive cluster-formers in C; this enzyme reaction separates them from the usual enterococcal branch. The anaerobic spore-forming rods in B suggest clostridia, whereas enterococci are non-spore-forming cocci. Coagulase-positive cocci (E) favour Staphylococcus aureus, since coagulase testing detects a characteristic plasma-clotting reaction. The combination in A instead identifies the classic pneumococcal pattern: paired cells with alpha-haemolysis, partial red-cell breakdown that gives green discolouration on blood agar, plus optochin sensitivity and bile solubility. Enterococcal identification uses a different combination of tests.