Emergence of antimicrobial resistance

Antibiotic exposure

The observation that changes in the incidence of bacterial resistance often mirror prior changes in exposure to antibiotics, has withstood experimental, epidemiologic, and mathematical scrutiny in both animals and humans. Hence, there is an enduring consensus that antibiotic exposure lies at the root of the complex mechanisms through which resistance emerges. The evidence linking antibiotic exposure to resistance is particularly strong for VRE. Most healthy human non carriers administered a glycopeptide antibiotic (Vancomycin or teicoplanin) orally will become colonized with VRE. There is also a consistent association between previous use of vancomycin, the VRE carrier state, and VRE bacteremia. While VRE colonization rates are twofold to ninefold higher in patients who have received vancomycin, prior exposure to this drug is neither required nor sufficient for colonization: third-generation cephalosporins, aminoglycosides, aztreonam, ciprofloxacin, imipenem and the anti-anaerobe antimicrobials clindamycin and metronidazole , have all been independently associated with the VRE carrier state. Antibiotic exposure can cause the emergence of VRE by inducing the expression of resistance genes and by selecting strains already expressing these genes. By altering the competing microbiota in the gastrointestinal tract, thereby increasing VRE concentration in the stools, antibiotic exposure can also facilitate the transmission of VRE.  

Recently, attention has focused on enterococci, not only because of their remarkable role in nosocomial infections, but also due to their remarkable and increasing resistance to antimicrobial agents. These two factors are mutually reinforcing since resistances allow enterococci to survive in an environment in which antimicrobial agents are heavily used. Antimicrobial therapy for enterococcal infections is complicated. Due to intrinsic low-level of resistance in enterococci to many antibiotics (clindamycin, aminoglycosides and β-lactams) a bactericidal effect cannot be reached at clinically relevant concentrations.

Traditionally, treatment of infections caused by enterococci has consisted of a synergistic combination of an aminoglycoside and a cell wall active antibiotic (e.g. ampicillin and vancomycin). However, emergence of resistance to these antibiotics has become a problem in many parts of the world. Antimicrobial resistance can be divided into two general types, that which is an inherent or intrinsic property and that which is acquired. The terms inherent or intrinsic resistance are used here to indicate resistance which is a usual species characteristic present in all or most of the strains of that species.

Antimicrobial resistance can be divided into two general types, that which is an inherent or intrinsic property and that which is acquired. The genes for intrinsic resistance, like other characteristics of the species, appear to reside in the chromosome. The various intrinsic traits exhibited by enterococci include resistances to semisynthetic penicillinase-resistant penicillins, cephalosporins, low levels of aminoglycosides, low levels of clindamycin and polymyxins.

The intrinsic resistance of enterococci to many commonly used antimicrobial agents may have endowed them with a cumulative advantage for further acquisition of genes encoding resistance to tetracycline, erythromycin (plus the newer compounds azithromycin and clarithromycin), chloramphenicol, high levels of trimethoprim, high levels of clindamycin, high levels of aminoglycosides, penicillin (by means of penicillinase), fluoroquinolones and vancomycin. Acquired resistance results from either a mutation in the existing DNA or acquisition of new DNA.

There are at least three major causes for the emergence of multidrug resistant enterococci:

1. Baseline that which is an inherent or intrinsic property to several antimicrobial agents.

2. Acquired resistance via mobile resistance genes on plasmids and transposons, chromosomal exchange, (and transfer of resistance to other bacteria including enterococci).

3. Mutations which lead to higher resistance.

Nosocomial transmission and risk factors

Spread in hospitals

Several studies have demonstrated the ability of resistant strains of enterococci to prevail and spread in hospitals. Enterococci are able to colonize not only the gut but also the skin, oral cavity and lungs of hospitalized patients. 

The intrinsic robustness of E. faecalis may allow members of this species to survive for extended periods of time, leading to its persistence and nosocomial spread. E. faecalis can grow at 10 to 45 ºC, in 6.5 % NaCl, in the presence of 40 % bile salts and over a broad range of pH. 

Originally, enterococcal infections were thought to arise from a patient’s own endogenous microbiota or to be introduced into the abdomen during transplant surgery or its complications . 

However, molecular epidemiological studies provided evidence for epidemic spread of enterococci in a hospital setting and nosocomial acquisition of enterococci. Long duration of stay in hospitals, stay in units with high proportions of colonized patients, use of electronic thermometers, and diarrhea have all been factors associated with spread of resistant strains. All these experiences make obvious the importance of having efficient infection-control routines in hospitals. Use of surveillance, isolation and barrier precautions has been successful in controlling minor outbreaks in non-endemic situations. 

Livornese et al. were the first to document an inanimate object, in this case rectal thermometer probes, as the mode of transmission of a vancomycin resistant E. faecium. Removal of the rectal thermometer probes resulted in termination of the outbreak. These reports were followed by many reports on nosocomial outbreaks and transmission of antibiotic resistant enterococci. The hands of health care workers are efficient tools for spreading microorganisms between patients in hospitals, this is true also for resistant enterococci. The impact of hand-washing has not been uniformly positive in US studies, possibly because use of gloves sometimes seems to have replaced hand-washing. Half a minute of wash with 60% alcohol solutions was more effective in eradicating VRE from the hands than was soap and water. E. faecium isolates survive for 7 days on counter tops, 24 hours on bedrails, 60 minutes on telephones and 30 minutes on stethoscopes. Rectal thermometers and even blood pressure cuffs have been involved in the spread of VRE. One recent study showed the environment to be contaminated around VRE colonized patients, another that the hands of medical staff may be equally easy contaminated with VRE by environmental contacts and colonized patients.

These findings illustrate the importance of proper cleaning of devices shared between patients and the use of hand disinfectants both before and after patient contact when trying to control spread of resistant bacteria in hospitals. Recently it was demonstrated that VRE could be controlled in a large region with 32 health care facilities, provided that consequent and active infection control measures including surveillance cultures and isolation of all infected and colonized patients were carried out. In that region, the overall prevalence of VRE decreased from 2.2% to 0.5% within 2 years.

On the other hand, if smaller outbreaks are not controlled the situation may become complex and even endemic. Bonten and coworkers studied VRE epidemiology in an ICU and introduced the term “colonization pressure” meaning the proportion of colonized patients in a setting during a given period. They found that if the colonization pressure was >50% all other measures had only little impact on the time to acquisition of VRE. In endemic setting measures such as protection of high risk groups, reduction of the total antibiotic pressure and education of staff may be more important than surveillance and isolation of every patient.

Risk factors for the nosocomial acquisition of enterococci that are described in the majority of studies are: previous antimicrobial therapy, duration of hospitalization, severe underlying disease, or invasive procedures. Nosocomial enterococcal acquisition and infection are often due to superinfection after the use of antibiotics with little or no anti-enterococcal activity like cephalosporins or quinolones.

Prevention and control of transmission include the controlled use of antibiotics, active surveillance cultures to identify the reservoir for spread and stringent application of recommended contact precautions. A variant of the esp gene was detected in all epidemic vancomycin-resistant E. faecium in hospitals, but not in non-epidemic animal isolates. This indicates that the surface protein Esp is associated with enterococcal colonization and spread. Analysis of the mechanism underlying the influence of this surface protein on enterococcal transmission might lead to new ways to prevent colonization and transmission.

Epidemiology of enecocci

Epidemiology

VRE has become a public health problem on the global perspective. It is clear that the epidemiology of glycopeptide resistance in enterococci is complex, with multiple factors contributing to its evolution and global dissemination. There are contrasting differences between continents and sometimes even between individual countries, depending on the resistance phenotype and genotype studied. Factors associated with these contrasting findings are associated with differences in the use of antimicrobial agents among humans and animals as well as differences associated with spread and colonization of individuals in different countries. Current studies have demonstrated the existence of major differences in the epidemiology of the spread of vancomycin resistance between the United States and Europe.

 United States

VRE in the United States seems to be a nosocomial problem, probably attributable to the extensive use of vancomycin and other broad-spectrum antibiotics. VRE were first reported in France in 1986, their occurrence has been reported from the U.S. in 1987. Since that time, VRE have been isolated from patients in Asia, Australia, and Africa. There is an epidemiological difference between the occurrences of VRE in the United States and in Europe. In the U.S. clones of VRE have spread within and between hospitals, but VRE among non-hospitalized humans have so far not been reported. Thus, VRE are thought to have evolved and spread due to the heavy antibiotic use in hospitals. In the United States, the percentage of nosocomial infections caused by VRE increased more than 20-fold (from 0.3% to 7.9%) between 1989 and 1993, indicating rapid dissemination.  From 1989-1993, the National Nosocomial Infection Surveillance (NNIS) surveys reported that the percentage of enterococcal isolates exhibiting vancomycin resistance increased from 0.3% to 7.9%, with a 34-fold rise seen in intensive care units (ICUs).  According to the NNIS System of the Centers for Diseases Control and Prevention (CDC), the proportion of VRE of the total number of enterococcal blood isolates increased from 13% to 26% between 1995 and 2000. Collecting information from more than 100 clinical U.S. laboratories, showed resistance to ampicillin and the glycopeptides to be rare in E. faecalis but very common in E. faecium, 83% and 52%, respectively. In 2003, the percentage of nosocomial enterococcal isolates exhibiting vancomycin resistance in ICU patients increased to more than 28%, an increase of 12% compared with 1998-2002.

 NNIS data reveal the pooled mean for vancomycin-resistant Enterococcus species from all ICUs, non-ICU inpatient areas, and outpatient areas were 13.9%, 12%, and 4.6%, respectively, from 1998 through June 2004. VRE was initially isolated mainly in large university hospitals, but subsequent reports demonstrate the presence of significant VRE epidemics in community hospitals and chronic care facilities, whereby a single clone can easily spread. VRE is isolated almost exclusively from hospitalized (or recently hospitalized) individuals.

In summary, the heavy increase of VRE in U.S. hospitals seems to be due to serious problems with both antibiotic overuse and infection control practices but there were no indications of input of VRE to hospitals from reservoirs in the community.

 Europe

Studies from European countries report a high prevalence of VRE, mainly E. faecium of the vanA genotype, among non hospitalized individuals, farmers, farm animals, in meat products, and in sewage treatment plants. There is now evidence to support the transmission of VRE to persons in contact with these sources, resulting in increased human reservoirs of VRE colonization. It was soon suspected that the widespread use of avoparcin for growth promoting purposes in farm animals might select for VRE among farm animals. Avoparcin confers cross-resistance to vancomycin of the vanA resistance genotype and this genotype has been dominant among both human and animal VRE isolates in Europe, whereas vanA and vanB have been equally prevalent in U.S. hospitals. Moreover, strains with identical transposons Tn1546 have been found in Europe among both animals and humans. Thus, there are several strong indications of the spread not only of resistant bacterial strains, but also of glycopeptide resistance genes from animals and via the food-chain to humans in European countries.

Southeastern Mediterranean

Despite the increasing reports of VRE in different countries, the reports of the   prevalence of VRE in Egypt, Jordan, Lebanon, are scarce. VRE in Israel seems to be a nosocomial problem.

In a prospective surveillance conducted to monitor the prevalence and dynamics of antimicrobial resistance among enterococci isolated from blood cultures in southern Israel. A total of 242 organisms isolated between 1993 and 1996 were studied. The prevalence of E. faecalis significantly decreased during the study period, whereas that of E. faecium doubled. Antimicrobial drug resistance increased steadily among E. faecium isolates: resistance to ampicillin increased from 19% in 1993–1994 to 53% in 1995, and to 67% in 1996. During the same period, resistance to vancomycin increased from 0% to 20%, and to 50%, and combined resistance to ampicillin and vancomycin and high-level resistance to gentamicin from 0% to 20% and to 38%.

 Other countries

 Saudi Arabia

A prospective study to determine the prevalence of its fecal carriage in patients at a tertiary care center in Saudi Arabia was conducted. During the period from March 1, 1995, to February 29, 1996, stool specimens examined from 4276 patients for the presence of VRE. VRE were found in six patients and all were identified as E. faecium. High resistance to vancomycin (MIC >256 µg/L) was found in all the six isolates, and to gentamicin in five isolates

 Kuwait

A study conducted to investigate the prevalence of antibacterial resistance in enterococci isolated from clinical samples in five hospitals in Kuwait. This study investigated the species prevalence and antibacterial resistance among enterococci isolated in Kuwait hospitals. They consisted of 415 isolates of E. faecalis (85.3 %), E. faecium (7.7 %), E. casseliflavus (4.0 %), E.avium (1.2 %), E. durans (1.0 %), E. gallinarium (0.5 %) and E. bovis (0.2 %) isolated from urine (36.6 %), blood (10.4 %), wound swabs (11.0 %), stool samples (12.0 %), high vaginal swabs (9.0 %), endocervical swabs (3.0 %) and miscellaneous sources (18.0 %). All of them were susceptible to linezolid. Fifty-two (12.5 %) isolates were ampicillin resistant but none of them produced ß-lactamase. The resistance to vancomycin was 2.6 %. All of the vancomycin-resistant strains carried the vanA phenotype and genotype. There was no evidence of clonal spread of the vancomycin-resistant isolates

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.