English: A

Beta-hemolysis and pigment production in eight strains of S. aureus on Columbia sheep blood agar. 30 μl of bacterial suspension, 0.5 °McFarland, inoculated on the agar plate. Cultivation 24 h, 37°C. Seen with reflected (left) and reflected + transmitted light (right). All tested strains are beta-hemolytic though in strains S1 and S4 is beta-hemolysis less conspicuous.

B

DNases are enzymes that hydrolyze DNA and release free nucleotides and phoshpate. DNase test is performed by incubating the isolates for 24 hours at 37°C on DNase agar (left), and pouring an excess (~15 ml) of 1 N HCl. Excess acid is removed with a pipette and clear zones around the bacterial colonies indicated DNase positive isolates (right seen with transmitted light and a black background). All six strains of S. aureus produce enzyme deoxyribonuclease.

C S. aureus cells, unlike S. epidermidis, form clumps when they are mixed with rabbit (or human) plasma. Presence of the clumping factor, termed bound coagulase, can be detected rapidly in the slide test. Coagulase is an enzyme-like protein and causes plasma to clot by converting fibrinogen to fibrin. This test requires several colonies of the tested strain and some other coagulase-negative staphylococci can also give a positive reaction using the assay (e.g., S. lugdunensis, S. schleiferi subsp. schleiferi). Positive agglutination results for the identification of S. aureus should be confirmed with other tests (e.g., growth and mannitol fermentation on mannitol-salt agar).

D Agglutination tests are very useful in rapid identification of staphylococci. If latex particles are coated with IgG and with human or animal fibrinogen, a staphylococcus will agglutinate if either clumping factor (binds to fibrinogen) or protein A (binds to the FC moiety of IgG) is present in the bacterial cell wall. Positive agglutination results should be confirmed with other tests or observations (some coagulase-negative staphylococci , including S. saprophyticus, possess protein A).

E

Staphylococcus aureus and S. epidermidis on mannitol salt agar. This medium is used for the selective isolation, cultivation and enumeration of staphylococci from clinical specimens. Staphylococci grow in the presence of 10% NaCl. S. aureus ferments mannitol and turns the medium yellow (phenol red is used as the pH indicator), S. epidermidis (and most other staphylococci) will grow but does not ferment mannitol. Mannitol salt agar usually contains ~7.5 %-10 % of salt, making it selective for gram positive bacteria. This level of NaCl is inhibitory to most other medically important bacteria.

F

Some pathogenic staphylococci change the opacity when grown in media containing egg yolk. Baird‐Parker medium is a selective medium commonly used for the isolation of S. aureus. The presence of egg yolk in this medium permits the detection of two reactions due to lipolytic activity of staphylococci: lecithinase reaction, a zone of precipitate in the medium surrounding the colonies and lipase reaction, an iridescent film (pearly layer) in and immediately surrounding colonies, visible by reflected light (iridescent sheen). Lithium chloride and tellurite are used as selective toxic chemicals in this medium. Staphylococci produce dark gray to black colonies due to tellurite reduction. Upper part of the plate S. aureus, underneath S. epidermidis and the close-up of S. aureus colonies with the lipase and lecithinase activity.

G

A wide variety of microorganisms produce enzymes capable of degrading hyaluronate. Among staphylococci is this ability restricted only to a few species. This activity can be seen in picture 'G'. Streptococcus equi subsp. zooepidemicus producing high quantity of hyaluronate was inoculated on the plate of blood agar. Tested strain of staphylococcus (yellow pigmented, surrounded by a zone of beta-hemolysis) was inoculated across and the plate was incubated for 18 hours at 37 °C. Decapsulation of Streptococcus equi subsp. zooepidemicus is visible in the vicinity of the tested strain (loss of the mucoid growth).