Staphylococcus epidermidis: A Commensal Emerging As A Pathogen With Increasing Clinical Significance Especially In Nosocomial Infections
M Rasheed, M Awole
biofilm, contaminant, neonates, nosocomial infections, prosthetic devices
M Rasheed, M Awole. Staphylococcus epidermidis: A Commensal Emerging As A Pathogen With Increasing Clinical Significance Especially In Nosocomial Infections. The Internet Journal of Microbiology. 2006 Volume 3 Number 2.
While at one time the appearance of
dismissed as contamination, it is now one of the most important agents of
nosocomial infections especially in immunocompromised individuals, neonates and
patients with internal prosthetic devices. In these patients, the infection result from a compromise in both general and local defense mechanism, and inability to clear staphylococci from infected device because of a biofilm on the foreign body surface.
Distinguishing, clinically significant pathogenic strains from contaminant
strains is one of the major challenges facing clinical microbiologists. Because many isolates are multiply antibiotic resistant, their infections are difficult to treat and can even be fatal.
A detailed characterization of isolates of
speciation, genetics and antibiotic susceptibility may be necessary to
distinguish infecting from contaminating isolates and to plan suitable therapy.
Coagulase negative staphylococci (CNS) constitute a major component of the normal skin and mucosal microflora1.
They are commonly isolated in clinical specimens and many are identified as important agents of hospital acquired infections especially in immunocompromised individuals, neonates and patients with internal prosthetic devices2,3.
CNS were considered as harmless skin commensal and dismissed as culture contaminants, but in recent years, they are increasingly being recognized as important human pathogens 1.
Among all CNS,
Molecular mechanism of biofilm formation
A characteristic of many pathogenic strains of
Recently, the genetic control of the slime production has begun to be elucidated, first in the
It appears that bacterial adherence is a complex multistep process that is influenced by the host, the device and the microbe. The adherence process is sub divided into the following stages: attachment, adhesion and aggregation8.
The primary attachment of
The later phases of adherence, in which organisms adhere to one another and elaborate biofilm, are mediated by polysaccharide intercellular adhesin (PIA), which is synthesized by products of the chromosomal ica gene locus, which comprises intercellular adhesion genes (ica A, ica D, ica B, and ica C) organized, in an operon 11,12, 13,14.
Mutants lacking PIA are less virulent in an animal model for foreign body infection and immunization with purified PIA is protective15.
Sub inhibitory concentrations of tetracycline and the semi synthetic streptogramin antibiotic quinupristin-dalfopristin were found to enhance ica expression 9 to 11 fold, whereas penicillin, oxacillin, chloramphenicol, clindamycin, gentamycin, ofloxacin, vancomycin and teicoplanin had no effect on ica expression. A weak (2.5 fold) induction of ica expression was observed for sub inhibitory concentrations of Erythromycin16.
According to the Centers for Disease Control and Prevention's National Nosocomial infection surveillance system; S. epidermidis is responsible for 33.5% of nosocomial blood stream infections18.
These bacteremias are largely due to intravascular associated infection. Unfortunately, nosocomial bacteremia due to
A study has demonstrated that the isolation of CNS was attributed to the colonization of the implanted catheter since the same microorganism had been isolated from the blood of patients during the preceding weeks, some of them with multiple positive cultures21.
It is known that mucosal damage of the alimentary tract and concurrent colonization of mucous membranes are risk factors for
Bacteremia originating from these sites can result from a compromise in both general and local defense mechanism in severely immunocompromised patients.
A large proportion of nosocomial isolates of CNS are resistant to multiple antibiotics, including penicllinase resistant penicillins24.
Localized infections with positive blood cultures are associated with higher mortality rate than localized infections without positive blood cultures25.
Infections associated with medical devices
In one study it was demonstrated that two clones of
At insertion, these clones constituted 13% CNS isolated from air samples and 44% CNS isolated from skin cultures.
After insertion, their combined prevalence increased to 53% in catheters not associated with CRI and 74% in catheters associated with CRI.
A likely source of
As 94% of these two predominant strains were ciprofloxacin resistant and all patients received selective antimicrobial prophylaxis with ciprofloxacin, these strains possessed a selective advantage.
Perhaps sub inhibitory concentration of ciprofloxacin is able to promote adherence of these two
However, in earlier studies, adherence of a variety of CNS strains was reduced after incubation with sub inhibitory concentration of ciprofloxacin28.
Migration of skin organisms at the insertion site into the cutaneous catheter tract with colonization of the catheter tip is the most common route of infection for peripherally inserted, short-term catheters 29,30.
Some catheter materials have surface irregularities that enhance the microbial adherence. Additionally, certain catheter materials are more thrombogenic than others, a characteristic that also might predispose to catheter colonization and catheter related infection29.
Teflon or polyurethane catheters have been associated with fewer infectious complications than catheters made of polyvinyl chloride or polyethylene31.
Contamination of the catheter hub contributes substantially to intraluminal colonization of long-term catheters and implicated as an additional entry point leading to catheter related sepsis justifying local use of antibiotics in preventive control measures29, 32.
Rarely, catheter might become hematogenously seeding from another focus of infection 29, 30.
It might be possible to eradicate methicillin resistant CNS at the site of catheter implantation by using antiseptics and to prevent ingress of CNS by a combination of occlusive dressings and careful handling of catheters and insertion sites with gloved hands22.
Coagulase negative staphylococci, usually
The vast majority of CNS causing PVE, when speciated were
In contrast, when infection involves native valves, only 50% of isolates were
Prosthetic infection can also be acquired from an infected intravascular device.
Nosocomial staphylococci tend to be multiple resistant.
Community acquired endocarditis, which may involve native (usually) or prosthetic valves, is increasingly recognized. Most patients with native valve infection have a pre-existing cardiac abnormality.
The organism must derive from the patient's skin but predisposing skin lesions are seldom detected. The infection often mimics
The commonest pathogen is
Some cases of endocarditis following implantation of a prosthetic valve were recently shown to be attributable to polyclonal
Complications, such as dehiscence of the valve or obstruction, are relatively common.
Blood cultures are usually positive and diagnosis is occasionally difficult. Antibiotic treatment of prosthetic valve endocarditis due to
Urinary tract infections
It is cultured almost exclusively from the urine of hospitalized patients with complications of the urinary tract. About half of them have an indwelling urinary catheter.
Both males and females are equally affected and most patients are 50 or more years of age. In at least half of the cases, the organisms are multiple drug resistant41.
Because of its ubiquitous nature and relatively low virulence,
Given the high level broad spectrum activity against most bacteria and the reduced frequency of ocular toxic effects, ciprofloxacin is currently considered the drug of choice in the therapy for bacterial keratitis45.
However, there is growing evidence for ciprofloxacin resistant ocular strains of
Vancomycin and teicoplanin are antistaphylococcal antibiotics, to which resistance is rarely seen, and should be considered the drugs of last resort for the therapy of nosocomial gram positive infections47. The multiple resistance of
Major advances in perinatal and neonatal care units have significantly improved survival of very low birth weight infants. However late onset nosocomial neonatal septicaemia, after more than 72 hours post delivery, by CNS , the most common organism accounting for more than 50% cases, show multiple antibiotic resistance including resistance to methicillin 48, 49.
CNS are ubiquitous and every human is colonized soon after birth: during invasive procedures these organism may then gain entry to the blood and result in sepsis50.
There is a clear co-relation between very low birth weight and the risk of a nosocomial infection with CNS51.
The intensive use of antibiotics in an NICU setting with highly susceptible patients causes selection of multiresistant clones of CNS, which subsequent becomes endemic52.
Quantitative biofilm production significantly greater in strains isolated from either the blood or skin of neonates with
Over the last decades, there has been an enormous increase and emergence of CNS strains particularly
Most of these strains harbor mec A, the gene encoding the penicillin –binding protein PBP2a, which has decreased affinity for beta lactam antibiotics57.
Detection of resistance to oxacillin in staphylococci is important to guide the therapy and prevent the patient from being unnecessarily treated with vancomycin, which is an antimicrobial agent that presents therapeutic complications, high costs and may lead to the selection of resistant mutants59.
In an article of Jukka Hyvarinan et al, the percentage of
Immunocompromised patients are particularly at risk of CNS infections, as are individuals with indwelling catheters or prosthetic devices.
Because many isolates are multiply antibiotic resistant, their infections are very serious and can even be fatal.
The surfaces and materials such as floors and walls of the hospital rooms, stethoscopes, Beds, tables for nursing elements, oxygen tube extreme and oxygen masks61, may play an important role in the spread of infectious agents including antimicrobial resistant strains of
The early and precise detection of these organisms in hospital environments and in high risk patients such as cardiac surgery patients, preterm newborns or immunocompromised patients, can prevent the contamination of prosthetic devices or indwelling catheters, and may represent a substantial help for the early treatment.
Rasheed M.U Lecturer, School of Medical Laboratory Technology Medical Faculty, Jimma University, P.O Box NO.378 Jimma, Ethiopia. Mobile: 00251 911 052 572 Phone No. 00251 47 111 8567-Residence 00251 47 111 1875- Office Email id: email@example.com