Staphylococcus species - Introduction

Introduction

There are a large number of Staphyloccocus spp present in the environment and many are commensals of the skin and mucous membranes of animals and man. Many are opportunistic pathogens causing pyogenic infections as a result of trauma, immunosuppression and secondary to other infections.

Endogenous or exogenous infections
Coagulase-positive S. aureus and S. intermedius as well as S. hyicus are important pathogens of animals
Fairly stable in environment
Strains selective for particular species
Cause mastitis, tick pyaemia, exudative epidermitis, botryomycosis and pyoderma

Characteristics

Although individual species may vary slightly, most have similar characteristics.

Gram-positive
Form grape-like clusters
Moderate sized, white/golden colonies
Facultative anaerobes
Grow on non-enriched media
Non motile
Catalase-positive
Haemolytic, 4 haemolysins recognised
Oxidase-negative

Detection & Differentiation

The most simple method of detection is to gram stain a sample, usually puss, and to then visualise the distinctive clusters under a microscope. This provides a basic identification of the type of bacteria, however occasionally it is necessary to identify the species that causing the infection. This can be done using various techniques based on the different characteristics of the species. Selective agar such as blood, for haemolysin producing species such as S. aureus and S. intermedius as well as non haemolytic species such as S. hyicus. MacConkey agar can be used to identify. PCR is the most specific method of determining the species of bacteria but requires laboratory analysis of samples and is only usually used in research.

Coagulase-negative strains vary in ability to cause haemolysis
Slide test to detect bound coagulase (clumping factor) on surface of bacteria; bacteria clump within 1-2 minutes
Tube test detects free coagulase, secreted by bactera; positive result indicated by clot formation in tube following 24-hour incubation; free coagulase converts prothrombin to thrombin which converts fibrinogen to fibrin

Pathogenesis and pathogenicity

Cause suppurative lesions
Trauma or immunosuppression predispose to infection
Capsular polysaccharide, teichoic acids and potein A prevent opsonisation and therefore phagocytosis
Cell wall proteins bind fibronectin and fibrinogen, allowing bacteria to attched to damaged tissues
Coagulase, DNase and protein A production mark pathogenicity
Haemolysins made by S. aureus and S. intermedius act as toxins
Alpha-haemolysin prouces a narrow zone of complete haemolysis; this toxin causes necrosis and targets mast cells and plasma cells, whose contents cause damage
Beta-haemolysin produces a wide zone of incomplete haemolysis; damages membranes and causes leakage, contributing to necrosis
Also gamma and delta toxins
Toxic shock syndrome toxin acts as a superantigen, which causes T cell proliferation and production of cytokines, leading to cardiovascular shock, with microthrombus formation in capillaries
Enterotoxins produced by some strains of S. aureus cause food poisoning in humans
Proteases, hyaluronidases and lipases facilitate survival of bacteria and spread and tissue destruction

Diagnosis

Clusters of bacteria in Gram-stained smears of pus
Culture on selective blood agar and MacConkey agar
No growth on MacConkey
Colony characteristics, haemolysis, catalase and coagulase production
Phage typing for epidemiological studies

Infections in dogs and cats

S. intermedius causes deep pyoderma, otitis externa, mastitis, endometritis, cystitis, osteomyelitis, wound infections, dyscospondylitis
S. aureus may cause gastroenteritis

Coagulase-negative staphylococci

Usually harmless commensals or secondary invaders
May adhere to indwelling catheters leading to urinary tract infections
Often display multiple antibiotic resistance

Enteritis

30% strains of Staphylococcus aureus produce potent enterotoxin. T
- Protein and heat-stable
- Responsible for staphylococcal food poisoning in man.
Every reason to assume that acute gastro-intestinal disturbance in small animals may be caused by these enterotoxins BUT not well documented.
Symptoms last 24-36 hours and include:
- Acute vomiting
- Diarrhoea
- Pain
The enterotoxins are superantigens.
- Induce release of cytokines from lymphocytes

Species specific infections

Small animals

S. intermedius causes deep pyoderma, otitis externa, mastitis, endometritis, cystitis, osteomyelitis, wound infections, dyscospondylitis
S. aureus may cause gastroenteritis
Bacteria localise in annulus fibrosis -> may spread to local bone in dogs

Sheep

Sporadically infect joints
May complicate 'tick-borne fever'

Pigs

Cause purulent arthritis
In one or more joints
Commonly contaminate wounds