Bacteriology

STAPHYLOCOCCUS AUREUS

Written by MicroDok

Staphylococcus aureus is a Gram-positive, coagulase-positive, catalase-positive, non-motile coccus found in the genus Staphylococcus and family Staphylococcaceae. They are facultative anaerobic organisms, and they cause haemolysis on blood agar. Staphylococcus species are usually arranged in groups, in pairs, tetrads and they also occur singly. S. aureus usually appear as grapelike clusters under the microscope. They are asporogenous or non-sporulating in nature. Asporogenous bacteria are organisms that do not produce spores. S. aureus are habitually found in the nose of humans but it may be found regularly in most other anatomical sites of the body such as the respiratory tract, mucous membranes, GIT and skin. S. aureus is mostly implicated in human pyogenic infections such as boils, pimples, impetigo and pustules.

S. aureus appears as a coccus, with a purple colour under the microscope.

PATHOGENESIS OF STAPHYLOCOCCUS AUREUS INFECTION

S. aureus is notorious in causing a variety of invasive and mixed infections in humans including pus-forming infections, food-poisoning (gastroenteritis) characterized by vomiting, skin infections and blood-borne related diseases. Urinary tract infections (UTIs), pneumonia, mastitis, endocarditis, meningitis and osteomyelitis are some of the serious infections or diseases in which S. aureus is implicated as a causative agent. The pathogenesis of S. aureus is based on the virulence factors (i.e. enzymes and toxins) that they produce.

LABORATORY DIAGNOSIS OF STAPHYLOCOCCUS AUREUS INFECTION

The laboratory diagnosis of staphylococcal disease is based mainly on the isolation and identification of the invading pathogen through microscopy and culture. Serological and biochemical tests (e.g. catalase, DNase and coagulase tests) are also employed in typing the strain of S. aureus implicated in the disease process. Blood, CSF, sputum, tracheal aspirate, pus, and surface swab specimens from infected sites (e.g. wound and burns) are clinical specimens collected for laboratory investigations. Gram staining reveals Gram-positive grape-like cocci in clusters, tetrads or pairs under the microscope.

S. aureus produces grape-like clonies that can either be in clusters or in tetrads on blood agar.

Mannitol salt agar (MSA) is a selective medium (that contains NaCl which inhibit other normal flora and non-staphylococcal organisms) used to screen for S. aureus and recover the pathogen from specimens resulting from a mixed infection. S. aureus produces several types of haemolysis including beta-haemolysis, alpha haemolysis and gamma haemolysis on blood agar media. S. aureus can also be cultured and isolated successfully on blood agar producing white or pale haemolytic colonies, chocolate agar and MacConkey agar. S. aureus grow aerobically at 35-37oC.

TREATMENT OF STAPHYLOCOCCUS AUREUS INFECTION

Therapy for staphylococcal disease is based on the administration of specific class of antibiotics to which the pathogen is susceptible to. All isolated Staphylococcus species should be subjected to antimicrobial susceptibility studies so as to guide treatment. Staphylococcal food poisoning should be treated through fluid and electrolyte replacement by the administration of the correct amount of a salt-sugar-solution (SSS) to the affected patients since the disease normally leads to a considerable loss of fluid from the body.

PREVENTION AND CONTROL OF STAPHYLOCOCCUS AUREUS INFECTION

Staphylococcus species are habitual inhabitants of the human body especially the nares and skin where they are resident as normal microflora. Most individuals who harbour pathogenic S. aureus are asymptomatic, and they shed the pathogen to susceptible hosts around them. The control and prevention of staphylococcal infections in the hospital environment should be based on the practice of proper hospital infection control measures such as hand washing and disinfection.

REFERENCES

Prescott L.M., Harley J.P and Klein D.A (2005). Microbiology. 6th ed. McGraw Hill Publishers, USA.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

Balows A, Hausler W, Herrmann K.L, Isenberg H.D and Shadomy H.J (1991). Manual of clinical microbiology. 5th ed. American Society of Microbiology Press, USA.

Barrett   J.T (1998).  Microbiology and Immunology Concepts.  Philadelphia,   PA:  Lippincott-Raven Publishers. USA.

Basic laboratory procedures in clinical bacteriology. World Health Organization (WHO), 1991. Available from WHO publications, 1211 Geneva, 27-Switzerland.

Murray P.R, Baron E.J, Jorgensen J.H., Pfaller M.A and Yolken R.H (2003). Manual of Clinical Microbiology. 8th edition. Volume 1. American Society of Microbiology (ASM) Press, Washington, D.C, U.S.A.

Murray P.R, Baron E.J, Jorgensen J.H., Pfaller M.A and Yolken R.H (2003). Manual of Clinical Microbiology. 8th edition. Volume 2. American Society of Microbiology (ASM) Press, Washington, D.C, U.S.A.

Murray P.R., Rosenthal K.S., Kobayashi G.S., Pfaller M. A. (2002). Medical Microbiology. 4th edition. Mosby Publishers, Chile.

 

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