Colonization vs Infection

Colonization vs Infection

Colonization is the presence, growth, and multiplication of the organism without observable clinical symptoms or immune reaction.

 1. MRSA – Colonization may occur in: the nares; axillae; chronic wounds or decubitus ulcer surface; perineum; around gastrostomy and tracheostomy sites; in the sputum or urine; and on healthy skin. One of the most common sites of colonization in both patients and employees is the nose (anterior nares). While healthcare workers may become colonized with MRSA (as they may with susceptible S. aureus), they rarely develop infections.

 2. Enterococci – Are normally found in the bowel, the female genital tract, and the mouth. Strains resistant to vancomycin (VRE) may survive and multiply, resulting in a colonization of the bowel.

 3. C. difficile – Commonly found in the gastrointestinal tract, the organism including drug-resistant and “epidemic strains” can asymptomatically colonize the bowel of individuals. Patients receiving antimicrobial therapy may be especially susceptible to developing CDI. Generally, there are more asymptomatic carriers than CDI patients. Though no symptoms may be evident, the colonized patient may test positive for the organism or its toxin(s).

 4. Multidrug-Resistant Gram-Negative Bacilli (MDR-GNB) – Colonization may occur on the skin (healthy skin and wounds) and the respiratory tract of both healthcare workers and patients. Colonization may also occur in the bowel where these organisms may occur as normal intestinal flora. As with other MDROs, infection of healthcare workers is rare.

        a. A. baumannii – Colonization may occur on multiple areas of the skin including the axillae and groin, as well as the respiratory tract of both patients and healthy individuals. Patients may also be colonized in wounds and occasionally the bowel. Colonization is particularly heavy during outbreaks.

       b. K. pneumoniae and other Enterobacteriaceae – May colonize wounds, healthy skin, the bowel, and the respiratory tract of patients and healthcare workers.

Infection refers to the invasion of bacteria into tissue with replication of the organism. Infection is characterized by isolation of the organism accompanied by clinical signs of illness such as fever, elevated white blood count, purulence (pus), and clinical expression of disease such as pneumonia, bloodstream infections, urinary tract infections, gastrointestinal infections, and skin infections.

Vancomycin-Resistant Enterococci (VRE)

 Vancomycin-resistant enterococci were initially reported in 1986 in Europe. In the last two decades enterococci have become recognized as a leading cause of healthcare associated bacteremia, surgical wound infection, and urinary tract infection. According to the National Nosocomial Infection Surveillance System (NNIS), prior to 1990 the occurrence of VRE infections in ICU’s in the U.S. was less than 1% of all enterococcal infections reported; by 1993 the occurrence had risen to 13.6% and ten years later, in 2003, VRE infections had more than doubled to 28.5%. Though the occurrence of VRE in hospitals was typically associated with larger hospital size (more than 200 beds) and university affiliation, hospitals of other sizes have also reported increases in endemic rates and clusters of VRE colonization and infection, indicating the upward trend is not limited by institution size. Data reported to the CDC during 2004 showed that VRE caused about one of every three infections in hospital intensive care units. This increase poses several problems, including the lack of available antimicrobials for therapy, since most VRE are also resistant to multiple other drugs (e.g., aminoglycosides and ampicillin) previously used for the treatment of infections due to these organisms. Many VRE are resistant to all presently available antibiotics. Several case-control and historical-cohort studies show that the risk of death associated with antibiotic-resistant enterococcal bacteremia is several times higher than the risk of death associated with susceptible enterococcal bacteremia.

 In addition, evidence suggests the vancomycin-resistant gene (VAN A gene) present in VRE may be transmitted to other gram-positive organisms, such as S. aureus. Though VRE is neither more infectious nor more virulent than susceptible enterococci, it poses a greater challenge because treatment options are limited to combinations of antimicrobials or experimental compounds with unproven efficacy.

Methicillin-Resistant Staphylococcus aureus (MRSA)

 Methicillin-resistant S. aureus is a variant of S. aureus which is considered to be resistant to all beta-lactam antibiotics (including penicillins, cephalosporins, and cephamicins). It may also be resistant to one or more other classes of antibiotics. By definition, MRSA must be resistant to one of the following semi-synthetic penicillins: methicillin, oxacillin, or nafcillin. Treatment of MRSA infections should be based on the susceptibility results from the patient culture. MRSA strains have been identified as a major source of healthcare-acquired infections and outbreaks in the U.S. For over four decades, MRSA has presented a challenge for infection control departments of hospitals attempting to control and eradicate this organism. In recent years, long-term acute care hospitals, long-term care facilities, rehabilitation centers, and small community hospitals have seen increasing numbers of cases. These facilities experience continuous reintroduction of resistant organisms due to the recurrent admissions and transfers of patients within these settings.

 More recently, MRSA has also been increasing in the community in individuals without healthcare-associated risk factors. In a 2005 study of S. aureus in Florida outpatient settings, 49.7% of S. aureus isolates were reported to be MRSA (Kolar and Sanderson, 2007). The strains of these CA-MRSA infections are genetically distinct from the typical HA-MRSA commonly encountered in healthcare settings.

1. Healthcare-Associated MRSA (HA-MRSA) – Infection and colonization are typically seen in older individuals with one or more of the risk factors outlined later in this newsletter series. Resistance to multiple classes of antimicrobial agents is common.

 2. Community-Associated MRSA (CA-MRSA) – Community-Associated MRSAc ases are frequently seen in younger persons and involve skin and soft tissue infections. Outbreaks of these infections have been described in numerous populations including people found in correctional facilities (jails and prisons), sport teams, men who have sex with men, commercial fishermen, and minority populations. Resistance to multiple classes of antimicrobials is uncommon. The most common CA-MRSA strain in the United States, the USA300 strain, is routinely resistant to erythromycin. Many of the CA-MRSA infections may be effectively treated with good wound care with or without oral antibiotics, while more resistant strains may require intravenous vancomycin. Frequently, these community-associated cases have initially been misdiagnosed as spider bites. This misdiagnosis prevents timely treatment which may result in a progression of the infection and increased chance of transmission to others. Although genetic variation exists between the types of MRSA, the community-associated variant has been found in healthcare settings and is capable of causing invasive infections and serious complications. HA-MRSA has also been demonstrated in community populations. Since distinction requires laboratory testing, the two variants are most often characterized by their operational definitions.