Just two out of the three species of the Staphylococcus genus – S. aureus and S. epidermidis – have medical importance. Staphylococcus aureus is the pathogenic species, while Staphylococcus epidermidis seldom causes human infections, being an opportunistic pathogen in people with compromised self-defenses.

Resistance
Staphylococci are one of the most resistant non-sporeforming cocci. Some can stand a temperature of 60 ºC for 30 minutes and contact with a solution of phenol 1% for 15 minutes. They remain as viable cells when refrigerated or even desiccated for as long as several months. The majority of the samples of Staphylococcus aureus grows in the presence of high concentrations of sodium chloride (10 to 15%) and resists to the bile salts action.

A long time ago, Staphylococci were resistant to penicillin, but more recently, the development of antibiotic-resistant strains has caused a serious problem of public health. In general, the predominant strains are antibiotic-sensitive, but there is an increasing number of antibiotic-resistant strains at hospitals. About 80% of Staphylococcus isolated from infected patients at hospitals are resistant to penicillin, streptomycin, tetracycline, oxacillin and even methicillin. In fact, Methicillin-Resistant Staphylococcus aureus (MRSA), Oxacillin- Resistant Staphylococcus aureus (ORSA), and Staphylococcus aureus that are resistant to several antibiotics or Multiple-Resistant Staphylococcus aureus are denominated super-germs.

Metabolic Products
Staphylococcus aureus produces several filterable toxins when cultured in appropriate conditions, especially under a CO2-rich atmosphere. The different toxins produced are discussed in the next paragraphs.

Alpha (α)-hemolysin: it is highly active against erythrocytes of rabbits, causing lyses at 37 ºC. It is moderately active against erythrocytes of sheep and inactive against human erythrocytes. Since nearly all samples of Staphylococcus aureus, recently isolated from human infections produce α-hemolysin, the synthesis of this enzyme is a strong suggestion of pathogenicity, although it is not conclusive for identification purposes because Streptococcus also produces this enzyme.

Beta (β)-hemolysin: it acts on erythrocytes of sheep, cater and human beings, but not on rabbits’. This enzyme is much less toxic than α-hemolysin for laboratory animals and is produced both in aerobes or anaerobes conditions.

Gamma (γ) and Delta (δ) hemolysins: they were isolated and identified as metabolic products from Staphylococcus, but their reactions are quite different from the previous ones. They are not known at the same extent of α and β hemolysins.

Leukocidin: another filterable product from Staphylococcus, known for its destructive action against leukocytes.

Coagulase: it is synthesized by Staphylococcus aureus and not by Staphylococcus epidermidis. For this reason, the coagulase test is often used as a bacteria identification method in order to differentiate S. aureus from Coagulase-Negative Staphylococci (CNS). Nowadays, CNS represents one of the most common agents of clinical infections, along with S. aureus. Staphylococci produce a pro-coagulase that reacts with a co-factor in citrated or oxalated plasma of rabbits or humans, resulting in the active enzymatic agent responsible for the plasma coagulation. The production of coagulase is easily demonstrated for the coagulation of a mixture of Staphylococcus aureus with diluted plasma.


Fibrinolysin: it is another factor produced by some samples of S. aureus, causing digestion of human, dogs, mice and sheep fibrin.