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.