Molecular mechanism of biofilm formation

A characteristic of many pathogenic strains of S.epidermidis is the production of slime resulting in biofilm formation5. The slime is predominantly a secreted teichoic acid, normally found in the cell wall of the staphylococci6. This ability to form a biofilm on the surface of a prosthetic device is probably a significant determinant of virulence for these bacteria 7. Biofilm may serve to protect the organism from host phagocytic mechanism and improve the local nutritional environment8.

Recently, the genetic control of the slime production has begun to be elucidated, first in the S.epidermidis and then in Staphylococcus aureus. Synthesis of the capsular polysaccharide is mediated by the ica operon (intercellular adhesion gene cluster)9.

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 S.epidermidis to a polystyrene surface is related to a cell surface protein exhibiting vitronectin binding activity10.

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.

S.epidermidis strains from clinical materials appears to differ from saprophytic strains by the presence of the ica A and ica B genes, their capacity for phase variation, their abilities to adhere to polymer and autoaggregate and in their colony morphology on congo red agar17.

Bacteremia

S. epidermidis and other CNS are the most frequently reported pathogens in nosocomial blood stream infections1.

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 S. epidermidis is a rapidly increasingly problem 5, 19, 20.

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 S. epidermidis infections 22. Colonization of the gut and skin with antibiotic resistant S. epidermidis can follow the intensive use of oral and systemic antimicrobials23.

Bacteremia originating from these sites can result from a compromise in both general and local defense mechanism in severely immunocompromised patients.

S. epidermidis frequently contaminate blood cultures making their interpretation a major concern for clinicians and for microbiologists. Although, criteria such as, sepsis symptoms, the number of positive blood cultures from separate venipuncture or access site and the similarity of their antibiotic resistance profiles, are often considered 5.

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

S. epidermidis is the most prominent cause of intravascular catheter associated infection19.

In one study it was demonstrated that two clones of S. epidermidis were predominantly involved in colonization and subsequent infection in neutropenic hemato-oncologic patients in a setting with a high incidence of Catheter Related Infection (6.0/1000 catheter days), and a very high catheter removal rate for CRI, 70%, despite prompt treatment with vancomycin. They seem to have persisted for a period of at least 5 years in the hematology department26.

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 S. epidermidis strains involved in CRI appeared to be the skin flora in 75% of cases 26.

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 S.epidermidis clones, as has been described recently for S. aureus 27.

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.

Endocarditis

Coagulase negative staphylococci, usually S.epidermidis, previously a minor cause of native valve endocarditis, are an important cause of prosthetic valve endocarditis and nosocomial infective endocarditis. Nosocomial native valve infections generally result from infected intravascular devices; the affected valve may or may not have been previously abnormal33,34.

S. epidermidis, are the predominant cause of nosocomial prosthetic valve endocarditis, can be acquired in the theatre (or shortly thereafter) at the time of the original valve replacement and presents within weeks or more often diagnosed within 60 days after surgery (early onset) and the cause of 3 to 8% of native valve endocarditis cases, usually in the setting of prior valve abnormalities34, 35, 36.

The vast majority of CNS causing PVE, when speciated were S.epidermidis.

In contrast, when infection involves native valves, only 50% of isolates were S.epidermidis 35, 36.

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 S. aureus endocarditis with rapidly destructive valvular disease, neurological manifestation and concomitant vertebral osteomyelitis.

The commonest pathogen is S. epidermidis, but there are increasing reports of other species, particularly S.lugdunesis, which seems to be especially virulent. These community acquired strains are frequently penicillin sensitive37.

Some cases of endocarditis following implantation of a prosthetic valve were recently shown to be attributable to polyclonal S.epidermidis populations38. Therefore, the detection in samples from the same patient of S.epidermidis strains with different antibiograms does not necessarily indicate contamination of the samples during collection.

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 S.epidermidis is often inadequate and additional surgical intervention is required in most cases 39, 40.

Urinary tract infections

S.epidermidis is the predominant species cultured form the urine in significant numbers (104 cfu/ml), accounting for 80-90% of non S.saprophyticus CNS isolates.

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.

Eye Infections

S. epidermidis is commonly cultured from the conjunctive and lid margins of normal subjects 42.

Because of its ubiquitous nature and relatively low virulence, S. epidermidis has received so far little attention for its role in ocular infections. However, in different studies

S. epidermidis has been reported to play a significant role in several ocular external diseases such as chronic blepharitis and suppurative keratitis 43, 44.

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 S.epidermidis 46.

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 S. epidermidis is a recognized problem. It might possibly represent a response to prolonged treatment.

Pediatrics

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.

S. epidermidis distinct clones can become endemic in NICUs as long as a decade and nosocomial transmission plays an important role in S.epidermidis bacteremia53.

Quantitative biofilm production significantly greater in strains isolated from either the blood or skin of neonates with S. epidermidis bacteremia54.

Drug Resistance

Over the last decades, there has been an enormous increase and emergence of CNS strains particularly S.epidermidis, S.haemolyticus and S.hominis, resistant to the antibiotic methicillin, especially in nosocomial settings55. Methicillin resistance/multiple drug resistance has been documented more often in disease causing strains of S. epidermidis than in skin colonizing strains 56.

Most of these strains harbor mec A, the gene encoding the penicillin –binding protein PBP2a, which has decreased affinity for beta lactam antibiotics57.

Methicillin sensitive S. epidermidis strains should be treated with oxacillin or cefazolin or clindamicin. Methicillin resistance is equivalent to resistance to oxacillin, which is commonly used and extremely effective anti-staph drug, in fact the drug of choice. Methicillin resistant S.epidermidis strains should be treated with vancomycin58.

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 S.epidermidis isolates resistant to the 20 tested antibiotics was oxacillin (58%), penicillin (82%), amoxicillin/clavulanic acid (34%), cephalothin (4%), cefuroxime (31%), cefotaxime (20%), imipenem (46%), gentamycin (46%), tobramycin (57%), netilmicin (16%), ciprofloxacin (23%), ofloxacin (21%), erythromycin (36%), fusidic acid (27%), clindamycin (34%), chloramphenicol (19%), rifampin (4%), vancomycin (0%), co-trimaxazole 62%, trimethoprim (53%) 60.