Staphylococcus Aureus Basic Characteristics

In this staphylococcus aureus basic characteristics post we have briefly explained about characteristics of staphylococcus aureus, virulence, pathogenicity, diagnosis, treatment

Staphylococcus aureus the family Micrococcaceae includes the genus Staphylococcus. Staphylococcus is a typical flora of the skin and mucous membranes, yet it is responsible for staph infection in humans.

The term Staphylococcus comes from two Greek words: ‘staphyle’ which means grape cluster and ‘kokkos’ which means berry. The pathogenic bacteria Staphylococcus aureus causes pyogenic infections in humans.

Staphylococcus aureus

Image Source: https://en.wikipedia.org/wiki/File:Staphylococcus_aureus_VISA_2.jpg

Characteristics of Staphylococcus Aureus

Morphology

Staphylococci are gram-positive spherical cocci with a diameter of 0.8–1.0 m that are grouped in grape-like clusters. They’re not motile or sporing, and just a few strains are capsulated.

Characteristics of Staphylococcus Aureus: Morphology

Cultural Characteristics

They are facultative anaerobes and aerobes with an ideal temperature of 37°C and pH of 7.4–7.6. They thrive on the following media and exhibit the typical colony form.

Characteristics of Staphylococcus Aureus: Cultural Characteristics

Virulence Factors

Peptidoglycan → It activates complement &  induces  release of inflammatory cytokines.

Teichoic acid → it facilitates adhesion of cocci to the host cell surface.

Protein A → It is chemotactic, antiphagocytic, anticomplementary & induce platelet injury.

Amongst the more common toxins secreted by S. aureus are hemolysin, leukotoxin, exfoliative toxin, enterotoxin, and toxic-shock syndrome toxin-1 (TSST-1). Aside from toxins, staphylococcal virulence factors also include enzymes and surface proteins.

Pathogenicity

Mode of Transmission: Staphylococcus infections are transmitted by the following ways. It includes the following infections, which are as follows: Direct Contact, Fomites (inanimate objects). Airborne droplets (Sneezing or coughing).

Cutaneous infections

Wound (injury), burn infections (tissue injury caused by heat), pustules (A small elevated skin lesions containing pus), furuncles (boil forms around a hair follicle and contains pus), styes (a painful swelling of hair follicle at eyelids), carbuncles (painful cluster of boils of the skin), Impetigo (skin infection with vesicles, pustules which ruptures), pemphigus neonatorum (an auto immune diseases that affect skin and mucous membranes).

Deep infections

It includes Osteomyelitis (inflammation of bones), tonsillitis (inflammation of tonsils), pharyngitis (inflammation of pharynx) sinusitis (inflammation of sinuses), periostitis (inflammation of membrane covering bones), bronchopneumonia (inflammation of lungs), empyema (collection of pus in the body cavity), septicemia (blood poisoning caused by bacteria and its toxins), meningitis (inflammation of meninge), endocarditis (inflammation of endocardium), breast and renal abscess.

Food Poisoning

Staphylococcal food poisoning may follow 2–6 hours after the ingestion of contaminated food (preformed enterotoxin). It leads to nausea, vomiting and diarrhea. Nosocomial infection: S. aureus is a leading cause of hospital acquired infections. It is the primary cause of lower respiratory tract (LRT) infections and surgical site infections and the second leading cause of nosocomial bacteremia, pneumonia, and Cardiovascular infections.

Exfoliative diseases

These diseases are produced due to the production of epidermolytic toxin. The toxin separates the outer layer of epidermis from the underlying tissues leading to blistering disease. The most dramatic manifestation of this toxin is scalded skin syndrome. The patient develops painful rash which slough off and skin surface resembles scalding.

Laboratory Diagnosis

Microscopy

Gram stained smears of clinical specimens is done, where gram positive cocci in clusters were observed.

Culture

The collected specimen is inoculated on selective media-MSA and the media incubated at 37°C for 18–24 hours. Next day culture plates are examined for bacterial colonies, which are identified by gram staining, colony morphology and biochemical tests.

Catalase test

The genus Staphylococci are catalase positive. Test distinguishes Staphylococcus from Streptococcus (catalase negative).

Coagulase test

Coagulase test:  This test helps in differentiating a pathogenic strain from non-pathogenic strain.

Treatment

Benzyl penicillin is the most effective antibiotic. Cloxacillin is used against beta lactamase. Producing strains (β-lactamase is produced by few strains of S. aureus which cleaves β – lactam ring of penicillin). Vancomycin is used against MRSA (Methicillin Resistant Staphylococcus aureus) strains.

Topical applications

For mild superficial lesions, topical applications of bacitracin or chlorhexidine is recommended.

Control measures

Proper sterilization of medical instruments must be done. Intake of antibiotics must be taken under proper medical advice. The detection of source & carriers among hospital staff, their isolation and treatment should be practiced.

Further Readings

Reference