By Kirsten Thompson
Editor's note: To access a diagram, the "Descending Order of Resistance to Disinfectants," see the February print issue of ICT.
As healthcare workers know, pathogens can survive in the environment for significant periods of time. And if surfaces are not cleaned and disinfected appropriately, these pathogens can be passed on to patients and hospital staff and cause significant, potentially fatal, health complications.
For example, viruses such as norovirus, can survive on dry surfaces for three weeks. Bacteria such as Acinetobacter, Staphylococcus and the endospores of Clostridium difficile, can survive even longer, in some cases for more than five months.
While healthcare workers may be aware of the persistence of pathogens on environmental surfaces, the aim of this article is to provide insight into the physiology of specific types of pathogens, particularly how disinfectants work to control their survival, and possible transmission.
Descending Order of Resistance to Disinfectants
Our approach for this article is based on the Descending Order of Resistance, which categorizes organisms from the most resistant forms to the most susceptible. As in the list, we begin with the most resistant organisms as they are by nature the most difficult to combat. The list evolved from the Spaulding classification that was first published in 1957 to describe the different classifications of medical devices and the types of disinfectants that could feasibly accomplish the proper level of disinfection. In recent years, there have been more organisms added to the list, such as prions and parasites, as well as the development of new disinfectant chemistries, but the descending order of resistance is simply a guide and is dependent on many variables.
The Most Resistant: Bacterial Endospores
The most resistant organism in the healthcare environment is the bacterial spore. Gram-positive rods have the ability to form an endospore, or an inactive form characterized by a waterproof cell wall that protects it from being dried out or damaged. The endospore coat is extremely resistant to environmental stresses such as drying and extreme heat, as well as chemical disinfection. For example, alcohol has been demonstrated to keep the endospore in the spore form, though quaternary ammonium compounds have been linked to an effect on germination (when the spore becomes an active and growing vegetative cell).
The most prominent bacterial spore in the healthcare environment today is Clostridium difficile (C. difficile). C. difficile is a gram-positive, anaerobic bacillus spore forming organism. It is active only when there is no oxygen present, such as in a person’s intestinal tract. The vegetative (actively growing) form of C. difficile is capable of producing toxins that cause C. difficile Infection (CDI) symptoms such as such as diarrhea, abdominal pain, fever and increased white blood cell count. When exposed to air, or any environment not suitable for its growth and reproduction, the vegetative form becomes dormant and forms an endospore.
In its endospore form, C. difficile can remain viable on surfaces for several weeks, even months. To be re-activated, the spore needs only to be ingested. It then migrates through the stomach and small intestine before arriving in the colon, where the environment is ideal for growth. The spore will then germinate to begin to grow and reproduce.
The vegetative form of C. difficile is as susceptible to disinfectants as other bacteria in hospitals, but C. difficile spores are much harder to inactivate because of their resilient spore coat. Disinfection should be carried out in two steps:
1. Surfaces must be thoroughly cleaned of all visible soil.
2. A surface disinfectant approved by the EPA for control of C. difficile endospores must be applied according to the directions on the manufacturer’s label. It is critical that the directions on the label be followed exactly in order for the C. difficile endospores to be killed.
There are few classes of disinfectants that are EPA-registered as sporicidal against C. difficile. Only oxidizing chemistries such as peroxygen compounds (peracetic acid, hydrogen peroxide), and bleach are aggressive enough to compromise the spore coat and inactivate the organism.
Mycobacteria are rod-shaped aerobic bacteria of the genus Mycobacterium, such as M. tuberculosis. Mycobacteria are among the most resistant organisms to environmental disinfectants because of their waxy outer layer which classifies it as an “acid-fast” bacterium. While some species are a concern with regard to bronchoscopes and other invasive medical devices, there is little concern for the transmission of mycobacteria from a hard surface.
Prior to the issuance of the Occupational Safety and Health Administration (OSHA)'s Bloodborne Pathogen Standard in 1990, this organism was included on disinfectant labels as a difficult to kill “marker organism.” Since bloodborne viruses such as human immunodeficiency virus (HIV) and hepatitis B virus (HBV) were not able to be tested against disinfectants at that time, this highly resistant organism was chosen to be a surrogate. Now that we understand the susceptibility of the bloodborne pathogens and are able to test them against disinfectants, the need for a mycobactericidal product has diminished.
Small, Non-Enveloped Viruses
The small, non-enveloped viruses such as noroviruses are extremely resistant to most disinfectants. Despite the lack of a lipid envelope, these organisms have a very resistant viral capsid which is made out of protein. The protein capsid is resistant to both lipophilic (oil-loving) disinfectants (i.e. quaternary ammonium compounds) as well as solvents (i.e., alcohol). Because of this, the norovirus is highly infectious and the second most frequent cause of acute gastrointestinal infections. Noroviruses have very high resistance levels and can remain infective for several months in a healthcare environment. Both this high resistance to environmental conditions and the small amount needed for an infection – as few as 10 to 100 virus particles are sufficient to trigger an infection – may explain how it spreads so quickly and widely.
Fungi are the next least resistant category of microorganisms. The fungi include mold and yeast, with mold spores being the most resistant form. Typically associated with moisture and damaged building materials, these organisms raise concern for the severely immunocompromised patients.
In descending order of disinfectant resistance, the Gram-negative bacteria are next. Gram-negative bacteria have an outer membrane made of lipopolysaccharide and proteins, which the Gram-positive bacteria are lacking. More multi-drug resistance is being recognized in this class of bacteria than previously, and they are becoming a formidable foe in the environment because of the lack of new antibiotics to treat infections.
In particular, Acinetobacter baumanii has been a big concern because it is, by nature, an environmental organism that is found in both soil and water. It can survive for several months in the environment and outbreaks are now being reported more frequently.
Extended-spectrum beta-lactamases (ESBL)-producing gram negative bacteria are also a threat in the environment because these bacteria produce enzymes that are able to inactivate the effect of beta-lactam antibiotics. Of these emerging pathogens, the Klebsiella pneumoniae carbapenemase-producing (KPC) is becoming a concern in healthcare facilities, with several outbreaks reported in the past few years.
Large, Non-Enveloped Viruses
Large, non-enveloped viruses are the next least resistant organism to disinfection. Although they have a resistant protein capsid, their larger size (70-100 nm) makes them more vulnerable than their smaller counterparts. Rotaviruses fall into this category and are easier to inactivate on an environmental surface.
Gram-Positive, Vegetative Bacteria
Gram-positive, vegetative bacteria are an even less resistant category of microorganisms. In contrast to the gram negative bacteria, gram positive organisms don’t have an outer membrane, making them more vulnerable to disinfectants. Examples include Staphylococcus aureus, including MRSA (both healthcare and community-associated strains) and Enterococcus sp, including Vancomycin-resistant Enterococci (VRE). These organisms are commonly found in the patient environment, easily spread and cause some of the most frequent and deadly healthcare associated infections. Despite the difficulty in treating diseases caused by multi-drug resistant gram positive organisms, most environmental disinfectants are capable of inactivating them on a hard surface.
The Most Susceptible: Enveloped Viruses
The enveloped viruses are the most susceptible to environmental disinfectants. The structure of these viruses includes a lipid envelope, which is easily compromised by most disinfectants. Once the lipid envelope is dissolved the core is exposed and vulnerable. Examples include the bloodborne pathogens, such as HIV, hepatitis B virus (HBV) and hepatitis C virus (HCV).
Human influenza A, another lipid-enveloped virus, is a concern for environmental contamination because of the many ways it can be transmitted, including: contact transmission, whether direct (touching an infected person) or indirect (touching an object that an infected person touched); droplet transmission (large droplets which are generated by sneezing, coughing or talking); and airborne transmission (due to small droplet nuclei). Despite the serious nature of these organisms and the diseases they cause, they are relatively easy to inactivate with environmental disinfection.
Antibiotic Resistance and Disinfectant Resistance
Although there has been much concern about antibiotic resistant organisms also becoming resistant to disinfectants, they are often no more so than their antibiotic-sensitive counterparts. This is due to disinfectants targeting multiple areas of the organism physically and disrupting cell processes. In addition, although organisms can be forced into resistance to disinfectants in the laboratory, there have not been any “captured” in the wild. The following table describes the differences between antibiotics and disinfectants, as well as their modes of action against microorganisms.
When disinfection resistance is forced in the laboratory, it is done so with repeated sub-lethal concentrations of the disinfectant. While this may occur with many different types of disinfectants, there are some disinfectants that are more likely to develop forced resistance than others. For this reason, having the right concentration is important for environmental disinfection. The proper concentration should be verified at the point of dilution, but also in the delivery to the environmental surface.
Selecting a Disinfectant
Lists of EPA-approved disinfectants for some of the pathogens of concern are available on the EPA website, but it should be noted that these lists are not updated frequently and not all organisms are represented. For example, a list of C. difficile sporicides is not available but many are represented by the parent registration which can be valuable in verifying an appropriate disinfectant.
Disinfectants: One Piece to the Environmental Hygiene Puzzle
Still, knowing which pathogens are persistent and what disinfectant works against them is just the first step in improving environmental hygiene and preventing their transmission.
It is also important to make sure there is a sound cleaning approach in place to ensure that all surfaces are being effectively cleaned. A recent study evaluated surfaces at 23 hospitals and found that, on average, only 49 percent of all terminally cleaned high-touch objects were effectively cleaned.
Environmental hygiene initiatives should be focused on a programmatic approach to cleaning the environment that includes the right products, tools, processes and improvement strategies to consistently decrease the bioburden on high-touch objects.
Kirsten Thompson is a microbiologist and technical affairs expert with Ecolab.