Basic Microbiology .1 Staphylococcus .1 Staphylococcus

The genus name Staphylococcus was derived from a Greek term staphylos which means a

‘bunch of grapes’, coccus means grain or berry (Figure 1.1). ‘Staphylococcus’ therefore implies that the cells of these organisms grow in clusters resembling clusters of grapes.

However, in clinical specimens, the organisms may also appear as single cells, pairs, or in short chains. The genus contains over 30 different species and only three of these are of clinical significance: S.aureus (causes a wide range of major and minor infections in humans and its enzyme coagulase causes clotting of blood plasma), S.epidermidis (usually a skin commensal bacterium, causes opportunistic infection associated with prostheses or foreign body) and S. saprophyticus (causes urinary tract infection in healthy adult women). (28, 29)

The staphylococci organisms are 0.5-1.5 µm in diameter, non motile, facultative anaerobic (i.e. grow in both aerobic and anaerobic environments), and are able to grow in media containing a high concentration of salt i.e. 10% sodium chloride as well as in temperatures ranging from 10-40^oC. Staphylococci are present on the skin and mucous membranes such as nasopharynx of humans and because shedding of this organism is common, it is responsible for the occurrence of nosocomial transmission among hospitalised individuals. (28)

Figure 1.1 Staphylococcus aureus resembling grain or berry-like in clusters like bunch of grapes (Adapted from www.microbiologyinpictures.com/staphylococcusaureus.html).

S.aureus is a Gram positive coccus, about 1 μm in diameter. The organisms are non-spore forming, non-motile, and usually non-capsulate. The organism is non fastidious, capable of aerobic and anaerobic respiration. S. aureus causes the following clinical conditions:

bacteraemia, osteomylitis, skin and soft tissues infection, pneumonia, toxic shock syndrome, surgical wound infection, and toxic epidermal necrolysis among others.

The treatments of choice for infection caused by S.aureus are penicillinase stable penicillins, since over 80% of hospital isolates are beta-lactames producers. (29, 30) The other challenge for treatment is methicillin resistance which has shown to be >30% in South African hospitals. (31) For these, vancomycin is indicated, but unfortunately emergence of vancomycin resistant S. aureus has been observed. (Chapter 6, table 6.2) The high rates of methicillin resistant isolates in many hospitals, is a major public health issue due to clinical implications of managing MRSA in the face of high resistance of first line treatment. This calls for active enhancement of surveillance of antimicrobial resistance among hospital isolates of S. aureus bacteria.

1.4.1.2 Klebsiella

Organisms belonging to the genus Klebsiella are capsulated, Gram negative rods, non-motile, approximately 1-2 µm in length (Figure 1.2). The capsule gives the mucoid appearance of isolated colonies and enhances virulence of organisms in vivo. Klebsiella genus belongs to the Enterobacteriaceae family, and the organisms are aerobic or ‘facultatively anaerobic’, ferment glucose and produce catalase but not oxidase. Hence the species and genera of the family of Entrobacteriaceae can be distinguished from each other by using biochemical tests in the clinical microbiology laboratory. The most commonly isolated members of this genus are K. pneumoniae and K. oxytoca, the latter being occasionally encountered in clinical specimens. (29, 30). The organisms grow at temperatures between 12^oC to 43^oC, and are found in the normal flora of the mouth, skin, and intestines.

Figure 1.2 Microscopic appearance of encapsulated non-motile rod-shaped Klebsiella pneumoniae bacterium (Adapted from www.microbiologyinpictures.com/klebsiellapneumoniae.html)

Klebsiella pneumoniae is clinically the most important member of the Klebsiella genus of the Enterobacteriaceae. It is common cause of infections among immunocompromised hospitalised individuals. The organism causes among others the following illnesses: severe bronchopneumonia, bacteraemia and meningitis, and milder wound and urinary tract infections etc. Mortality associated with illness such as bacteraemia is high. (29) However, the main relevance of this micro-organism in humans is that it commonly causes surgical wound infections, urinary tract infection and bacteraemia among hospitalised individuals.

The challenge in clinical management of these micro-organisms is related to plasmid

mediated multiple antibiotic resistances, limiting the choice of effective antimicrobial agents.

(30) Regular surveillance to determine patterns of antimicrobial resistance would assist to guide empirical treatment and improve treatment outcomes.

1.4.1.3 Pseudomonas aeruginosa

This micro-organism is non-spore forming, motile by means of polar flagella, non-capsulated, straight or slightly curved Gram-negative rods typically arranged in pairs (Figure 1.3). The size of the micro-organisms measures 0.5 to 0.8 µm by 1.5 to 3.0 µm. The micro-organisms are saprophytic and found mostly in soil, water, and other moist environments. P. aeruginosa is an opportunistic pathogen and patients usually become infected through contact and spread if exposed to environmental sites colonised by these bacteria. The micro-organism typically produces a blue green pigment (pyocyanin) and a yellow-green pigment (pyoverdin). (28-30)

Figure 1.3 Appearance of Pseudomonas aeruginosa on Gram stain: rod-shaped cells arranged in pairs (Adapted from Todar’s online textbook of bacteriology)

P. aeruginosa causes infections on the skin, burn wounds and nosocomial pneumonia in critically ill hospitalised individuals under intubation. In addition, P. aeruginosa can also cause bacteraemia, osteomylitis, endocarditis and urinary tract infection. P. aeruginosa is a common pathogen that causes nosomial infection among hospitalised individuals, and is a major lung pathogen among patients with cystic fibrosis. (30) Infections caused by

P.aeruginosa are clinically challenging to manage owing to intrinsic resistance to multiple antibacterial agents and the pathogen has a strong ability to acquire resistance from other antimicrobials during the course of treatment, particularly with prolonged broad spectrum antibiotics. (29, 32)

Owing to its inherent ability to develop resistance, effective monitoring patterns of resistance to antipseudomonal drugs would help to guide clinicians in their choice of antibiotics for empirical management of such infections, thereby enhancing infection control to minimise continued spread of resistant strains.