Genus Characteristics

The staphylococci are Gram-positive cocci with an average diameter of 0.8 to 1 µm, that tend to be arranged in pairs, tetrads, or more often, grouped in irregular clusters or ‘bunches of grapes’ (Fig. 7.1). Colonies are usually white with regular edges. They are non-motile, non-sporulating and most species are facultative anaerobes, with a fermentative metabolism. They are sensitive to lysostaphin (MIC of 12.5 µg/ml) and furazolidone (100 µg/disc) and resistant to lysozyme (MIC of 1000 µg/ml), bacitracin (0.04 unit disc) and to O/129 (0.5 mg). They are usually catalase-positive and oxidase-negative. Growth occurs on nutrient and blood agars but not on MacConkey agar. They are usually not capsulated or have a limited amount of capsule. There are about 30 species of staphylococci and most are found in animals but few are pathogenic. They are considered opportunistic pathogens. Infections with staphylococci are often acute and pyogenic. The two major pathogenic staphylococci, Staphylococcus aureus and S. pseudintermedius are coagulase-positive. The coagulase test usually correlates well with pathogenicity. However, the cause of exudative epidermitis in young pigs, Staphylococcus hyicus, can be coagulase-negative as only 24 to 56% of isolates are coagulase-producing isolates. Coagulase-negative staphylococci occur as commensals and in the environment. They are considered a major component of the normal microflora of animals and humans and occasionally cause opportunistic infections. In this chapter, we will focus primarily on the identification of S. aureus, S. pseudintermedius, S. hyicus, S. chromogenes, S. aureus subsp. anaerobius, S. delphini, S. schleiferi subsp. coagulans and S. felis, the species most commonly associated with animal infections.

Staphylococci Compared with Other Gram-Positive Cocci

Micrococci are non-pathogenic, Gram-positive cocci that could be confused with coagulase-negative staphylococci. However, micrococci are variably positive to conventional oxidase tests, oxidase-positive in a modified oxidase test (Faller & Schleifer 1981), are oxidative in the O-F test and have a different susceptibility pattern to bacitracin and furazolidone. The colonies of the micrococci can be white but are often pigmented, the pigmentation ranging from a garish-yellow through cream, to buff or pink (M. roseus) (Fig. 7.2). Streptococci and enterococci are distinguished from staphylococci by the catalase test. Macrococci cells are approximately 4 to 5 times bigger than staphylococcal cells with a diameter of about 2 µm. The pertinent reactions for the commonly isolated Gram-positive cocci are summarized in Table 7.1.

Natural Habitat

Staphylococci are widespread in nature and occur worldwide in mammals and birds. They colonize the nasal cavity, naso-pharynx, skin, and mucous membranes. They can be transient in the intestinal tract. Many infections are endogenous but prolonged survival of staphylococci in the environment (some resistance to heat, NaCl and certain disinfectants) permits indirect transmission.

Pathogenesis and Pathogenicity

The staphylococci are pyogenic bacteria associated with abscess formation and suppuration. Pathogenic Staphylococcus species can infiltrate the tissues following a cutaneous lesion and produce suppurative lesions which usually remain localized at the infection site. Pus is composed of the debris of dead leukocytes and both living and dead bacteria. This is surrounded by intact phagocytic cells and fibrin strands. A fibrous capsule will eventually be formed around an abscess. In chronic staphylococcal wound infections (‘botryomycosis’) the lesion is granulomatous with pockets of pus throughout the tissue.

The pathogenic staphylococci produce a ‘battery’ of toxins and enzymes, but the significance of many of them in the pathogenesis of disease is not fully understood. Enterotoxins (A–E) are involved in human food poisoning (mainly enterotoxin A). They act by reflex stimulation of the emetic centre. Exfoliatin produces staphylococcal scalded skin syndrome (SSSS) in human infants and dogs. Human toxic shock syndrome (TSS) is caused by TSS toxin-1. Epidermolytic toxins are implicated in porcine exudative epidermitis and in staphylococcal skin conditions in humans and dogs. The alpha toxin (a haemolysin) is associated with gangrenous mastitis in cattle. This toxin is a pore-forming toxin and causes lysosomal disruption in leukocytes and also affects smooth muscle, leading to constriction, paralysis and finally necrosis of the smooth muscle cells of the walls of blood vessels. A leukocidin kills neutrophils and macrophages of cattle, rabbits and humans. Protein A, a surface component of most strains of virulent S. aureus, binds to the fragment crystallizable (Fc) region of IgG and may play a part in the pathogenesis of staphylococcal diseases.

Enzymes include staphylokinase which is a plasminogen activator; coagulase which causes plasma coagulation in vitro; hyaluronidase (‘spreading factor’); lipase; collagenase; proteases; nucleases and urease, all of which may have a role in the pathogenesis of staphylococcal infections. Table 7.2 lists the main diseases caused by the pathogenic staphylococci while their main virulence factors are presented in Table 7.3.

Species Characteristics

The species characteristics discussed in the paragraphs below are those usually used to identify these organisms in a diagnostic veterinary bacteriology laboratory. Key tests for rapid identification of the most clinically significant Staphylococcus species are found in Table 7.4 while a schematic representation is shown in Figure 7.3.