Staphylococcus aureus

Staphylococcus aureus is a gram positive coccus known to cause localized suppurative lesions. Von Recklinhausen  first observed Staphylococci in human pyogenic lesions in 1871. Later, in 1880, Sir Alexander Ogston, a Scottish surgeon conclusively established the role of Staphylococci as a causative agent in pyogenic lesions. Though the new edition of Bergey’s Manual of Systematic Bacteriology mentions more than 40 species of Staphylococci, Staphylococcus aureus still remains the most critical pathogen in the family Staphylococcaceae.

Staphylococcus aureus is resistant to decolorisation with organic solvents such as alcohol or acetone. Therefore it is categorized as gram positive. The virulence factors that contribute to disease are as follows:

• teichoic acids 
• capsular polysaccharide 
• peptidoglycans 
• Protein A 
• hemolysins 
• toxins 
• enzymes  
• superantigens.

Some of these virulence factors are responsible for the unique presentation of lesions caused by Staphylococcus aureus. The enzyme coagulase produced by Staphylococcus aureus encapsulates suppurative lesions producing localized abscesses. Similarly, superantigens produced by this organism cause a specific syndrome in humans known as Toxic Shock Syndrome. It is characterized by multisystem involvement. These staphylococcal enterotoxins (TSST-1) are capable of stimulating T lymphocyte proliferation regardless of antigenic specificity and result in large scale production of chemokines leading to a multisystem disorder involving the kidneys, blood vessels, mucosae, liver, skin and the central nervous system.

Drug resistance in Staphylococcus aureus

Resistance to penicillin by Staphylococcus aureus was observed soon after Sir Alexander Fleming discovered penicillin in 1928. This was later shown to be related to the production of certain enzymes called β lactamase (penicillinases) which are produced by the organism. These enzymes are capable of hydrolyzing the β lactam ring of penicillin thereby rendering it ineffective. The genes for β lactamase were acquired by plasmid mediated transfer from the environment. There are about four such penicillinases which have substrate specificity. Semisynthetic penicillinase resistant penicillins such as oxacillin, naficillin and methicillin,  were the drug of choice for treating infections caused by Staphylococcus aureus producing penicillinases.

The 1980s saw resistance developing to the penicillins. These organisms were denoted Methicillin Resistant Staphylococcus aureus (MRSA). This type of resistance is mediated through a cell wall protein called penicillin binding protein 2a (PBP-2a). The genetic basis of acquiring this resistance was the acquisition of chromosomal mecA gene which produces a defective penicillin binding protein 2a (PBP-2a) with low binding affinity to all β lactam agents. These include not only penicillins, but also the cephalosporins. Studies have also shown that genes determining resistance to other antimicrobial agents like clindamycin, macrolides and aminoglycoisdes may also be present in MRSA thereby making treatment of infections caused by to MRSA extremely challenging.

MRSA is often involved in the following

• surgical site infection 
• burn infections 
• pneumonias 
• catheter associated sepsis
• blood stream infections

MRSA is a hospital acquired, or nosocomial pathogen with 85% of invasive MRSA infections associated with healthcare and about one third of them occurring during hospitalization. In 2005, the Centers for Disease Control (CDC), estimated approximately 94,360 people to have developed serious MRSA infections, of which 18,615 people died during hospitalization in the United States. Surveillance has shown that strains of MRSA isolated from invasive infections have been associated with healthcare related procedures. The major risk factors for acquiring these strains in the hospital are the elderly (>65years), blacks and males. According to the CDC, approximately 25% to 30% of the population is colonized with Staphylococcus aureus with approximately 1% of them being colonized with MRSA.

Community acquired MRSA (CA-MRSA) infections are now being reported with increasing frequency from different parts of the world. These occur in otherwise healthy people who have not come in contact with any healthcare associated procedure within one year. They frequently present as painful boils and purulent pimples. These lesions often become severe and form large nonhealing wounds. Fulminant pneumonia and blood stream infections in otherwise healthy individuals are some of the sobering clinical presentations of CA-MRSA. The CDC has investigated clusters of such infections occurring among prisoners, military recruits, athletes and children. Some of the risk factors for acquiring CA-MRSA are poor hygiene, overcrowding, sharing of contaminated items and surfaces as well as other activities involving close skin to skin contact.

Cleaning and disinfection

Staphylococcus aureus tends to colonize the mucosal and skin surfaces of human beings. It is transmitted either by close skin to skin contact or when an individual comes in contact with contaminated inanimate objects such as towels, bandages, razors etc. Staphylococcus aureus is a powerful organism capable of surviving desiccation and extremes of temperatures for prolonged period of time. When dried on threads, they retain their viability for 3 to 6 months. Staphylococcus aureus has been isolated from dried pus even after 2 to 3 months. It’s survival also is contingent on the type of surface such as porous, nonporous, plastic, spongy etc.

Hand hygiene and cleaning of surfaces which come in contact with the skin are essential measures in prevention and control of MRSA colonization and infection. Hand hygiene encompasses hand washing with soap/detergent and water as well as use of alcoholic hand rubs. The CDC recommends the use of a thorough soap and water hand wash when the hands are visibly contaminated or soiled. Proper rinsing of the hand is an important step of this procedure. Use of alcohol based sanitizers should be a frequent practice among healthcare workers in order to prevent colonization and spread of infection between patients. These are quick (30 seconds to 1 minute contact time) unlike standard soap and water hand washes and are therefore feasible in clinical settings. The contact time specified by the manufacturer is critical due to the time required for the disinfectant to kill or inactivate 99.99% of the organisms.

Surfaces such as bench tops or patient tables also need to be cleaned and disinfected thoroughly. Most cleaners do not disinfect and most disinfectants do not clean. Cleaning is important in order to remove any obvious soil or dirt from the surface and must be done prior to disinfection for it to be effective. The CDC recommends the use of EPA registered disinfectants to be used for the purpose of disinfection. Some combinations of detergent/disinfectants are now available and the manufacturer’s instructions regarding contact time must be strictly followed.

Pathogens such as MRSA, whether community acquired or hospital acquired, can be easily taken care of utilizing strict hygiene precautions and infection control measures.