Pathogenicity of enterococci

 Endocarditis

Of the diverse infections caused by enterococci, infective endocarditis (IE) is one of the most therapeutically challenging. Enterococci are the third leading cause of infective endocarditis, accounting for 5-20% of cases of native valve IE, and 6-7% of prosthetic valve endocarditis.

As with other enterococcal infections, most isolates are E. faecalis; however, other species can also cause this disease. Among isolates sent to the Centers for Disease Control, endocarditis was the diagnosis given for patients from whom E. avium, E. casseliflavus, E. durans, E. gallinarum, and E. raffinosus, as well as E. faecalis and E. faecium, were isolated. This condition usually occurs in older patients. Their presentation is typically subacute. Usually, left-sided endocarditis and mitral valve involvement is more common than aortic involvement. Risk factors include urinary tract infection or instrumentation. The presence of the pheromone-responsive plasmid pAD1 enhances vegetation formation in enterococcal endocarditis.

 Enterococcal bacteremia

Enterococcal bacteremia is much more common than enterococcal endocarditis. Nosocomial surveillance data for the period October 1986-April 1997 list enterococci as the third most common cause of nosocomial bacteremia, accounting for 12.8% of all isolat.

The translocation of enterococci across an intact intestinal epithelial barrier is thought to lead to many bacteremias with no identifiable source. Other identifiable sources for enterococcal bacteremia include intravenous lines, abscesses, and urinary tract infections. The risk factors for mortality associated with enterococcal bacteremia include severity of illness, patient age, and use of broad spectrum antibiotics, such as third-generation cephalosporins or metronidazole. Community-acquired enterococcal bacteremia is more commonly associated with endocarditis (up to 36% of cases) than nosocomial bacteremia (0.8%).

Nosocomial enterococcal bacteremias may arise from a variety of sources. Polymicrobial bacteremias including enterococci and other bowel microbiota should increase the index of suspicion for an intra-abdominal source. Other sources may include surgical sites and burn wounds infections. Blood cultures that grow enterococci may be positive because of contamination of the skin with these organisms. A positive blood culture result for Enterococcus species in the absence of evidence of ongoing infection should raise this possibility.

 Urinary tract infection

The most common type of infection caused by enterococci is usually nosocomial (associated with urinary tract catheterization or instrumentation). The bladder, prostate, and kidney are commonly infected by enterococci, especially in patients with structural abnormalities of the urinary tract or indwelling catheters. Cystitis and pyelonephritis are common infections. Occasionally, prostatitis and perinephric abscesses may develop. Occasional infections may occur in young, healthy women (<5%) found in up to 15 % of urine isolates, ranking only second after E.  coli.

The clinical manifestations of enterococcal urinary tract infection are similar to those of other organisms. A reliable diagnosis of urinary tract infection can be difficult because enterococci are opportunistic pathogens that can also be colonizers or cause asymptomatic bacteriuria. Different studies were performed to investigate the role of surface proteins of E. faecalis in the interaction with uroepithelial tissue.

Neonatal infections

Although group B streptococci and E. coli are the most common causes of neonatal infections, it has been well documented that enterococci can also cause infection in this population .

Enterococcal meningitis is an uncommon disease accounting for only 0.3% to 4% of cases of bacterial meningitis which is nevertheless associated with a high mortality rate. It has been described most frequently in patients with neurosurgical conditions (i.e. head trauma, shunt devices, or cerebrospinal fluid leakage), although it can also occur as a "spontaneous" infection complicating remote enterococcal infections such as endocarditis or pyelonephritis. E. faecalis and E. faecium are the two species most frequently isolated during the course of meningitis (76%–90% and 9–22% respectively). E. casseliflavus can be inserted among the etiologic agents of meningitis. Awareness of infection of central nervous system with Enterococcus spp. that possess an intrinsic vancomycin resistance should be increased .

 Central nervous system infections

In addition to causing neonatal meningitis, enterococci can also cause central nervous system infections in older children and adults. Most cases seem to be related to an underlying disorder. Enterococci have also been reported as a cause of central nervous system shunt infections, particularly those that terminate in the peritoneum.

 Intraabdominal and pelvic infections

Although enterococci can be isolated in a significant number of intra-abdominal infections, usually as part of a polymicrobial infection, their role in these infections is controversial. Animal models of bacterial peritonitis showed that enterococci alone did not cause any abscess formation, but a mixed inoculation of E. faecalis and other aerobe or anaerobe bacteria resulted in death and abscess formation suggesting a synergistic effect of E. faecalis in the pathogenesis of bacterial peritonitis This finding is underscored by the fact that antibiotics that lack activity against enterococcus can often be employed successfully in intra-abdominal infections, even when enterococci are present as part of the polymicrobial microbiota.  However, others suggested that the role of E. faecalis in experimental peritonitis might depend on the presence of virulence factors.

Despite the difficulty in establishing pure enterococcal infections, it is clear that enterococci can cause and contribute to abdominal and pelvic abscess and sepsis. Antimicrobial regimens with minimal in vitro antienterococcal activity are effective for treating mixed infections; therefore, the pathogenicity of enterococci in this setting is questionable. Antienterococcal bactericidal activity is recommended when blood culture results are positive for enterococci.

  Endophthalmitis

Colonization of host tissue may play a role in the pathogenesis of endophthalmitis. Enterococci are among the most destructive agents that cause this post operative complication of cataract surgery. Experiments designed to determine whether aggregation substance targeted E. faecalis to alternate anatomical structures within the eye showed that enterococci attach to membranous structures in the vitreous, but that such adherence is not dependent on the presence of aggregation substance.

Skin and soft tissue infections

E. faecalis accounts for up to 5 % of isolates from skin and soft tissue infections.  Enterococci generally cause infections only in previously damaged tissues and are not apparently responsible for primary cellulites. Especially in wound infections after abdominal surgery, enterococci are frequently cultured. However, since enterococci from skin and soft tissue infections are frequently cultured in association with other pathogens, their role in pathogenicity is unclear. Figure 2.1 showing necrotizing cellulitis due to VRE developed in the right thigh and lower abdomen in neutropenic patient.

Figure : Necrotizing cellulitis due to VRE developed in the right thigh and lower abdomen in neutropenic patient.