Breaking the Chain of Infection and Preventing Cross-Contamination

A plethora of studies in the medical literature has demonstrated that nearly everything in the healthcare setting – from surfaces, to healthcare workers’ hands, to medical equipment and everything in between — can serve as a reservoir and a vector for opportunistic pathogenic organisms. The acquisition of infections in the healthcare facility is aided by a number of important factors, including:

• The persistence of some bacteria and viruses on inanimate objects and surfaces for weeks and months (Kramer, et. al., 2006)

• The lack of hand hygiene and sanitation in healthcare facilities (Pittet, et al., 2000)

• Breaches in evidence-based infection prevention practices

• The growing volume of patients admitted in acute-care hospitals and increasing clinical acuity of immunocompromised patients (Johnston and Bryce, 2009)

• The growing shortage of healthcare professionals and the tendency to cut corners by existing staff

All of these factors can create that “perfect storm” where healthcare-acquired infections (HAIs) can brew, unless all healthcare workers understand and implement proper hygiene practices to help prevent the unintended transmission of bacteria and viruses.

The stakes are getting higher these days, with the list of significant hospital pathogens growing in length. Lisa A. Morici, PhD, research assistant professor at Tulane University Health Sciences Center’s Department of Microbiology and Immunology, says she is seeing everything from methicillin-resistant Staphylococcus aureus (MRSA) and methicillin resistant and susceptible Staphylococcus aureus (MSSA), to Pseudomonas aeruginosa, E. coli, Haemophilus influenzae, coagulase-negative Staphylococcus or S. epidermidis, Enterococcus sp., Streptococcus pneumoniae, Klebsiella pneumoniae, Candida species and Acinetobacter baumannii.

“The majority of these organisms are opportunistic pathogens in that they can only cause disease in immunocompromised individuals or when there has been a breach in natural host defenses,” Morici explains. “Thus, the greatest threat is to the patient. Threats include cross-transmission among patients and increased antibiotic resistance among these organisms leading to increased difficulty in eradication and treatment of infections. Risks to caregivers include colonization with antibiotic resistant organisms that can be transmitted to personal contacts or lead to endogenous infection when that person becomes immunocompromised.”

The Chain of Infection

One of the cornerstones of infection prevention and control practice is understanding the basic chain of infection. The links are: infectious agent, reservoir, portal of exit from the reservoir, mode of transmission, and portal of entry into a susceptible host. Transmission of infection in a hospital requires, at the very least, three of those links: a source of infecting microorganisms, a susceptible host and a means of transmission for bacteria and viruses.

An infectious agent is any disease-causing organism and includes bacteria, viruses and fungi; complicating matters further is the organism’s pathogenicity and virulence. A reservoir is any animate or inanimate object or surface where an organism can live, grow and multiply. A portal of exit is the path by which an agent departs the reservoir and can include body fluids or the mechanisms of coughing/sneezing. Modes of transmission are contact, airborne, ingestion or vectorborne. A portal of entry is the path by which an infectious agent enters the susceptible host and can include broken skin, eyes, nose, respiratory tract, urinary tract, etc. And finally, a host can be anyone, with some individuals being particularly susceptible – immunocompromised patients, the very young and the very old, etc.

“Hands, skin, and mucosal membranes of patients and care-givers serve as both reservoirs and vectors of transmission; water or moist environments such as toilets, sinks, and mops serve as reservoirs as well,” Morici says. “All of the most commonly isolated organisms colonize the hands, skin, or mucosal surfaces of all individuals or exist very well in moist environments.”

Human sources of microorganisms are healthcare workers, patients and visitors, any of whom may be individuals in the incubation period of a disease, those who already have a disease, or those who are considered to be chronic carriers of an infectious agent. Other sources of bacteria are the patient’s endogenous (produced or originating from a cell or organism) flora and inanimate objects that have become contaminated, otherwise known as fomites. While there are five main routes of transmission of bacteria — contact, droplet, airborne, common vehicle and vectorborne — the first three routes are most critical to the discussion of nosocomial infections.

Contact Transmission

Two modes of contact transmission exist: direct-contact transmission and indirect-contact transmission. Direct-contact transmission, as its name implies, consist of direct body surface to body surface contact and physical transfer of bacteria between a susceptible host and an infected or colonized individual. Examples of this kind of contact include bathing or turning a patient. Indirect-contact transmission involves contact of a susceptible host with a contaminated object such as medical instruments, dressings, gloves that are not changed between patients or unwashed hands.

Droplet Transmission

Infectious droplets are generated during coughing, sneezing, talking and through procedures such as bronchoscopy and suctioning. Transmission occurs when these droplets are propelled a short distance through the air and deposited on a host’s mouth, nasal mucosa or conjunctivae. Droplet transmission is not to be confused with airborne transmission because droplets do not remain suspended in the air and special air handling and ventilation is not required.

Airborne Transmission

This kind of transmission consists of dust particles containing bacteria or airborne droplet nuclei (small-particle residue 5 mm or smaller in size) of evaporated droplets containing microorganisms that are suspended in the air for longer durations of time. Microorganisms transmitted in this manner can be inhaled by a susceptible host, so special air handling is indicated to prevent the spread of infection. Smallpox and Mycobacterium tuberculosis are a few examples of bacteria spread by airborne transmission.

Prevention Best Practices

In theory, the prevention of cross-contamination is as simple as following evidence-based practices demonstrated to break the chain of infection. Morici’s list of practices includes: better hand hygiene practices; adoption of standardized line insertion and care practices; proper barrier precautions; improved catheter technology; shorter time periods for indwelling catheters. “Person-to-person transmission of these opportunistic pathogens can be reduced by handwashing, barrier use, and proper procedure,” she says. “Improved technology for catheters and medical implants that lead to reduced bacterial colonization and reduced biofilm formation can help prevent nosocomial infections from contaminated devices.”

Johnston and Bryce (2009) emphasize that four key hospital interventions — hand hygiene, environmental cleaning, barrier precautions and screening — are the cornerstones of infection control and prevention of cross-contamination. Johnston and Bryce (2009) note, “The hospital provides a suitable environment for an infectious agent to come into contact with a susceptible host. Any intervention that successfully breaks just one link in the host–environment–agent chain will interrupt transmission.”

Numerous studies also document the importance of the mechanical action and thoroughness of the cleaning effort. Johnston and Bryce (2009) observe, “Meticulous care must be taken to avoid cross-contamination of the environment via contaminated mop heads, cleaning solutions or cloths. The basic principles of keeping clutter to a minimum, cleaning from the top down and the use of fresh solutions in the correct concentration for adequate contact time are worth repeating. To do this properly, time and sufficient staff are required. The reflex to decrease the number of housekeeping staff to cut costs is short-sighted because it may be followed by an increase in more costly cross-transmission of hospital-associated microorganisms. Additionally, all healthcare workers need to understand that they have a responsibility for maintaining a clean environment and that this is not simply a ‘housekeeping’ issue.”

A proper environmental cleaning intervention has been shown to reduce the transmission of multidrug-resistant organisms (MDROs) in intensive care units (ICUs), according to a new study. Researchers found that following an enhanced cleaning protocol reduced the spread of MRSA to patients exposed to rooms in which the prior occupant had been colonized or infected.

The multi-modal cleaning intervention consists of three parts: a change from use of a pour bottle to bucket immersion for applying disinfectant to cleaning cloths; an educational campaign involving the environmental services staff at the hospital; and feedback method using removal of intentionally-applied marks visible only under UV light.

“We know that environmental contamination with highly antibiotic-resistant bacteria can still occur in hospitals where cleaning policies exceed national standards established by the CDC,” says Rupak Datta, MPH, of the University of California at Irvine. “Although the risk of acquiring MRSA and VRE is already low, this study suggests that there are additional preventive measures that hospitals can take to reduce the risk of transmission from one patient to another.”

The retrospective study of more than 13,000 hospital stays in10 ICUs at a large, tertiary care academic medical center in Boston, measured the risk of MRSA and vancomycin-resistant Enterococci (VRE) acquisition before (Sept. 2003 to April 2005) and during the cleaning intervention (Sept. 2006 to April 2008). Routine admission and weekly screenings for MRSA and VRE were conducted during both periods providing a systematic method to identify new cases of MRSA and VRE. During the pre-intervention period, 3.9 percent of the 1,454 patients exposed to a prior occupant with MRSA acquired the pathogen compared to just 1.5 percent of the 1,443 patients exposed during the intervention. Of the 1,291 patients exposed to VRE prior to the intervention, 4.5 percent acquired VRE compared to 3.5 percent of 1,446 patients during intervention.

The study builds upon a body of research conducted by Datta and his co-authors. In a 2006 study, they found that patients admitted to an ICU room whose prior occupant had been infected with MRSA or VRE had as much as a 40 percent increased risk of acquiring either pathogen, suggesting environmental contamination could play a significant role in their transmission. In a subsequent study, the authors showed that a multi-modal cleaning intervention could reduce environmental cultures for MRSA and VRE. The current study now suggests that this same intervention reduces acquisition of these pathogens, particularly MRSA, in subsequent room occupants.

Despite the overall reduction in MRSA and VRE acquisition, the cleaning intervention appears to be more effective against MRSA compared to VRE. The difference could be due to a generally higher rate of VRE room contamination and Datta suggests it is a question for future research. “The results suggest that a multi-modal cleaning intervention can reduce MRSA and, to a lesser extent, VRE transmission in high-risk patient areas including the ICU,” Datta says.

Datta and his co-authors point out that a relatively small percentage of healthcare-associated infections (HAIs) are transmitted due to inadequate room cleaning (constituting about 5 percent of all new cases of either pathogen.) Still, there are measures hospitals can take to put patients’ minds at ease. “Even though we know the risk is relatively low, it is unsettling for patients admitted to hospitals to know that the health condition of the prior room occupant could impact their risk for acquiring MRSA or another antibiotic-resistant infection.” Datta adds.

Barrier Precautions

The value of preventing exposure to potentially infectious material by barrier-protection items such as gloves, gowns, face masks and eye protection cannot be understated. Johnston and Bryce (2009) note, “Gowns, masks and eye protection are used to protect uncovered skin, clothing and mucous membranes from splashes or sprays of blood, body fluids, secretions and excretions. Under certain circumstances, when routine practice affords insufficient protection against the cross-transmission of microorganisms (to either health care workers or other patients), additional precautions such as “contact precautions” are required. The assumption is that patients or their environments, or both, are colonized and contaminated in the absence of blood, body fluids, excretions, secretions and visible soiling.” The researchers add further, “The use of contact precautions to prevent transmission of C. difficile, MRSA and vancomycin-resistant Enterococci has been met with reservation for a number of reasons. Until recently, many hospitals in the United States have resisted adopting these measures because of incongruencies in national and international guidelines regarding recommendations for use of contact precautions, the lack of high-quality studies to support their implementation, and, in particular, a lack of resources to apply these interventions quickly, efficiently and consistently.”


In a perfect world, Johnston and Bryce (2009) say, routine infection prevention and control practices would be enough to prevent cross-transmission of pathogenic organisms, but instead, they remark, “It is abundantly clear that this is not the case. If additional precautions are to be applied, it is imperative that the patients for whom these precautions are necessary be identified in a timely manner. This may be done by screening patients believed to be at risk for colonization with MRSA and vancomycin-resistant Enterococci ... Although some patients may clear their colonization with MRSA and vancomycin-resistant Enterococci, many remain colonized (persistently or intermittently) for many years. Most hospitals take a conservative approach and identify these patients as colonized for future admissions. Although studies are ongoing, the balance of evidence does not support routinely decolonizing patients with MRSA and vancomycin-resistant Enterococci.”

Johnston and Bryce (2009) note, “Experience and evidence have taught us that the core components of infection prevention and control are consistent application of proper hand-hygiene measures, maintenance of a clean environment, use of barriers where appropriate, and prompt identification of patients at high risk of colonization with a transmissible microorganism. These components apply to all infections, whether or not the pathogen is known. Gaps in knowledge exist, such as a complete and integrated understanding of the determinants and facilitators for hand hygiene, identifying which barrier precautions offer the most benefit for the least risk, and predicting situations in which screening is most cost-effective.”


Pyrek KM. Breaking the chain of infection. Infection Control Today. July 2002.

Kramer A, Schwebke I and Kampf G. How long do nosocomial pathogens persist on inanimate surfaces? A systematic review. BMC Infectious Diseases. 6:130;2006.

Pittet D, Hugonnet S, Harbarth S, et al. Effectiveness of a hospital-wide program to improve compliance with hand hygiene. Lancet 2000;356:1307-12.

Johnston BL and Bryce E. Hospital infection control strategies for vancomycin-resistant Enterococcus, methicillin-resistant Staphylococcus aureus and Clostridium difficile. Can Med J.180(6): 627-631. March 17, 2009.

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