Fomites and Infection Transmission


In a systematic review of the literature, German researchers explored the ability of infectious organisms to survive on inanimate surfaces.1 They found that most gram-positive bacteria, including vancomycin-resistant enterococcus (VRE), methicillin-resistant Staphylococcus aureus (MRSA), and Streptococcus pyogenes can survive for months on dry surfaces. In general, there was no obvious difference in survival between multiresistant and susceptible strains of Staphylococcus aureus and Enterococcus spp., the authors write. Only in one study was such a difference suggested, but the susceptible strains revealed a very brief survival as such. Many gram-negative species, such as Acinetobacter spp., Escherichia coli, Klebsiella spp., Pseudomonas aeruginosa, Serratia marcescens, or Shigella spp. can survive on inanimate surfaces even for months. These species are found among the most frequent isolates from patients with nosocomial infections. A few others, such as Bordetella pertussis, Haemophilus influenzae, Proteus vulgaris, and Vibrio cholerae, however, persist only for days. Mycobacteria including Mycobacterium tuberculosis and spore-forming bacteria, including Clostridium difficile can also survive for many months on surfaces.

Overall, the review noted that gram-negative bacteria persist for longer periods of time than gram-positive bacteria. Climactic factors can play a role in persistence as well humid conditions were found to increase survival times for most types of bacteria, such as Salmonella typhimurium, Pseudomonas aeruginosa, and Escherichia coli. Only Staphylococcus aureus was found to persist longer at lower humidities.

In most studies with experimental evidence, persistence was studied on dry surfaces using artificial contamination of a standardized type of surface in a laboratory. The authors explain, Bacteria were prepared in broth, water, or saline.

Viruses were usually prepared in a cell culture medium. The main advantage is that the environmental conditions are consistent regarding temperature and air humidity. In addition, the effect of temperature or relative humidity can only be determined under controlled conditions, which are much easier to ensure in the laboratory.

However, this may not always reflect the clinical situation, in which surfaces can be simultaneously contaminated with various nosocomial pathogens and different types of body fluids, secretions etc. Yet the question remains: what is the clinical evidence for the role of surfaces in nosocomial infections?2

In the hospital environment, surfaces with which hands come in contact are often contaminated with nosocomial pathogens, and may serve as vectors for cross-transmission. A single incidence of hand contact with a contaminated surface results in a variable degree of pathogen transfer, say the researchers. Transmission to hands was most successful with Escherichia coli, Salmonella spp., Staphylococcus aureus (all 100 percent); Candida albicans (90 percent); rhino virus (61 percent); hepatitis A virus (HAV) (22 percent to 33 percent); and rotavirus (16 percent). Contaminated hands can transfer viruses to five more surfaces or 14 other subjects. Contaminated hands can also be the source of recontaminating the surface, as shown with HAV. Compliance rates of healthcare workers in hand hygiene are known to be around 50 percent. Due to the overwhelming evidence of low compliance with hand hygiene, the risk from contaminated surfaces cannot be overlooked.3

Observational evidence has suggested that the environment may play a significant role in the transmission of nosocomial pathogens during outbreaks. This has been described for various types of microorganisms, including Acinetobacter baumannii, Clostridium difficile, MRSA, VRE, SARS, and norovirus. However, the authors emphasize that evidence to support the role of environmental contamination is not equally strong for all types of nosocomial pathogens.

For Clostridium difficile, MRSA, and VRE, data are stronger than for other pathogens, such as Pseudomonas aeruginosa or Acinetobacter baumannii, of which multiple types were detected in the environment, and which did not always correlate with the acquired strain, they write. Overall, the review supports current guidelines which recommend disinfection of surfaces in specific patient-care areas in order to reduce the risk of transmission of nosocomial pathogens from inanimate surfaces to susceptible patients.4

It is widely agreed that the potential role of fomites in the transmission of disease requires further study. A study from the University of Arizona sought to evaluate transfer efficiency rates of different types of bacteria and viruses from surface to surface.5

The study consisted of two evaluation periods. In the first, subjects hands were sampled following contact with one of eight common surfaces that had been inoculated with a pool of three microorganisms. In the second period, subjects lower lips were sampled after they had been touched with a fingertip that had been inoculated with a pool of the same three microorganisms.

The present study suggests that gram-positive bacteria are transmitted most readily from environmental surfaces followed by viruses and gram-negative bacteria, the authors write. In a study by Scott and Bloomfield (1990a), Staphylococcus aureus and Escherichia coli were transferred from a laminate surface to the fingertip at similar rates. Therefore, more research is needed to determine whether gram-positive bacteria are, indeed, transferred more readily than gram-negative bacteria or whether transmission rates from fomites are organism specific. Different survival rates may have influenced some of these results.6

It is clear that inanimate objects and the hospital environment can become contaminated with dangerous pathogens, and that these organisms can persist for long periods of time if not eradicated. Can fomites be a direct link to patient infection? Due to the many ways in which transmission is possible, and the many interventions employed in the case of an outbreak, consensus on this question remains elusive.

In this study, transfer efficiency from nonporous surfaces was calculated differently than from porous surfaces, due to the nature of the two types of surfaces. Porous surfaces can be inoculated with, and hold, a known volume of pooled bacteria and phage. Most of a measured volume of inoculum will run off of hard, smooth, curved, nonporous surfaces. Hence transfer efficiency rates must be calculated in a different manner for these two types of surfaces, the authors explain. Transfer rates from hard, nonporous surfaces were more efficient than from porous surfaces. A porous surface, such as a sponge, offers many deep recesses in which bacteria and viruses reside becoming less accessible to the human hand. A hard smooth surface does not offer crevices or passages in which microorganisms may hide, hence higher transmission. However, high levels of hand contamination occurred in spite of poor transfer rates from some of the porous fomites.7

The authors concluded that commonly handled objects which are microbially contaminated can serve as reservoirs of bacteria and viruses that can easily transfer to the hands through direct contact, which in turn can be easily transferred to the lip.

Numerous studies have assessed the possible role of fomites in outbreaks. In one instance, a prospective observation study compared MRSA and VRE cultured from nosocomial infections with MRSA and VRE cultured from enteral feeding tubes, both from a neonatal intensive care unit.8 DNA fingerprinting was used to compare the pathogens. The authors found that there were 23 S. aureus isolates; 12 of 23 were methicillin resistant. There were four MRSA infections in patients without feeding tubes. DNA fingerprinting showed that the MRSA species causing each of the clinical infections was also found in the feeding tubes of other babies. Based on this evidence, the authors concluded that feeding tubes are a reservoir for antibiotic-resistant pathogens that can be transmitted to other infants.

Blood pressure cuffs have also been scrutinized. One study assessed the level of bacterial contamination on blood pressure cuffs in use on hospital wards.9 Viable organisms were recovered from all the 24 cuffs sampled at a density of between 1,000 and more than 25,000 colony-forming units per100 cm2. Another study investigated the role of blood pressure cuffs in the spread of bacterial infections in 18 hospital units.10 Potentially pathogenic microorganisms were isolated from 27 (13 percent) of the 203 blood pressure cuffs evaluated. The highest rates of contamination were observed on cuffs used in intensive care units and those kept on nurses carts. For four patients with a personal sphygmomanometer, the authors found a genetic link between the strains isolated from the blood pressure cuffs and the strains isolated from the patients.

Computer equipment may be a culprit as well. A study examined the microbial contamination of computer user interfaces with potentially pathogenic microorganisms, compared with other fomites in a surgical intensive care unit of a teaching hospital.11 The authors acquired sterile swab samples from a patients bedside computer keyboard and mouse, and three other sites in the patients room in a 14- bed surgical intensive care unit. Samples from the keyboard and mouse of the physicians workstation, as well as control buttons of the wards intercom and telephone receiver, were obtained. Analysis from samples in the patients rooms yielded 26 contaminated samples from keyboard and mouse (5.9 percent) compared with 18 positive results from other fomites within patients rooms (3.0 percent).

At the physicians computer terminal, two samples obtained from the mouse (6.3 percent) showed positive microbial testing whereas the wards intercom and telephone receiver were not contaminated. The authors concluded that the colonization rate for computer keyboards and mice of patient data management systems with potentially pathogenic microorganisms is greater than that of other user interfaces in a surgical ICU. These fomites may then become vectors for cross-transmission of nosocomial infections in the ICU setting.

According to the work of Bala Hota MD, MPH, the quality of the evidence that examines contamination of inanimate objects and the environment should be judged according to whether four factors have been measured:12

  • The degree of contamination of the environment by specific pathogens 

  • Whether or not temporality is addressed (i.e., whether the environment is contaminated before or after patient colonization) 

  • The assessment of confounders, such as hand hygiene and the quality of cleaning of fomites 

  • Whether or not improved cleaning, after controlling for other interventions, reduces the risk of patient infection 

According to the author, The best studies of cross-colonization of patients from the inanimate environment use molecular epidemiologic techniques to identify pathogens, measure the quality of environmental cleaning and hand hygiene over time, and link contaminated surfaces and cross-colonization events in geographic and temporal dimensions.13

In terms of bacteria, environmental contamination with C. difficile has been reported to occur in areas near infected or colonized patients. Commodes, bedpans, blood pressure cuffs, walls, floors, washbasins, and furniture are commonly affected. The organism has also been found in low numbers on shoes and on stethoscopes, and hospital floors have remained contaminated with C. difficile for up to five months.14

A. baumannii has been isolated throughout the inanimate environment from the beds of colonized patients and on nearby surfaces (e.g., mattresses and bedside equipment), to hospital rooms (e.g., on floors, sinks, countertops, and door handles), and in room humidifiers. However, some studies have found no strains of the organism in the inanimate environment during outbreaks of infection with A. baumannii among patients, making the role of the environment unclear. But the levels of hand hygiene and environmental cleaning are not commonly reported in outbreak investigations, and it is possible that the importance of environmental contamination is confounded by other interventions.15

Contamination of fomites with VRE has been found on many occasions in the literature, including gowns worn by patients and healthcare workers, medical equipment, microsphere beds, and environmental surfaces. The degree of environmental contamination with VRE has also been shown to correlate with the number of body site that have been colonized with VRE.16 Transmission from surfaces to patients might occur: contact with contaminated surfaces alone is almost as likely to lead to contamination of the hands of healthcare workers as is contact with a colonized patient, Hota writes. Other data supporting environment- to-patient transmission demonstrate that noncolonized patients who were admitted to contaminated rooms had highly increased odds of acquisition of VRE.17 

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