Case of the Month

May 2012

Dr Andrew Whitelaw, NHLS, Groote Schuur Hospital

The setting

The case revolves around carbapenem-resistant Acinetobacter baumannii in a paediatric ICU. The ICU has a capacity of 18-22 beds (depending on staffing levels), and is situated in a tertiary academic hospital in the public sector. Table 1, below, shows the number of A. baumannii bacteraemias for 6 month periods in 2010 and 2011, as well as the carbapenem resistance rates.

Table 1. Acinetobacter bacteraemias 2010-2011, with percentage of isolates resistant to meropenem

The ICU, infection control team and hospital management had been actively trying to reduce the Acinetobacter resistance rates as well as the number of isolates. Measures included strict contact precautions for colonized / infected patients; environmental sampling; staff education; culturing staff hands; reviewing intubation and suctioning practices; antibiotic restrictions.

In July 2011, a review of the pasteurization unit at the hospital was performed. The ventilator circuits used in the ICU were being cleaned and pasteurized on site prior to re-use. At the review, a number of difficulties related to the cleaning of the tubes were identified, primarily that due to the length and nature of the tubes, cleaning them prior to pasteurization was extremely difficult, and there was sometimes residual moisture in the tubes after the drying process. As a result of this review the decision was made by the hospital management to use disposable ventilator circuits in the ICU, rather than re-usable ones.

Question 1: Where is Acinetobacter baumannii normally found?

Answer to Q1

A. baumannii is one of the non-fermentative Gram negative bacilli, often appearing as Gram-negative cocco-bacilli on Gram stain of clinical specimens. It was probably first recognized as a separate organism in 1911, although has undergone numerous changes to nomenclature since then. Although a number of Acinetobacter species have been described, A. baumannii (or members of the A. baumannii complex) is generally regarded as the species most commonly involved in clinical infections.

All Acinetobacter species (including A. baumannii) have been traditionally regarded as environmental organisms. They can thus be found in soil, water as well as arthropods and occasionally other environmental sources. There have been a number of reports of A. baumannii causing infection in soldiers wounded in combat (in Vietnam, as well as in the Middle East). While this was initially thought to be the result of contamination from environmental sources, this has not been conclusively proved, and some studies suggest that the Acinetobacters isolated from patients with combat injuries are in fact due to early nosocomial transmission. More recent evidence from environmental sampling studies suggests that while many Acinetobacter species are found in the environment, A. baumannii is found relatively infrequently.

Acinetobacter species, including A. baumannii, have been isolated in low numbers from fresh fruit and vegetables. This has raised concerns that hospital food may serve as a vehicle for transmission of A. baumannii, and some studies have shown digestive tract colonization to be high (up to 41% of ICU patients). Whether digestive tract colonization is directly linked to contaminated food has not been established.

Acinetobacter has also been detected in body lice, and just over 20% of 622 lice were found to harbor A. baumannii. Again, the clinical relevance of this finding is unclear.

Possibly more importantly, they can on occasion be colonisers of skin, although not to the same degree as the normal skin commensals such as coagulase negative staphylococci. Skin colonization may be more common in patients who are hospitalized rather than in the community. Studies of healthy volunteers have detected a range of Acinetobacter species on the skin in over 40% of the individuals; however A. baumannii itself was rarely detected (< 1%). However, in hospitalized patients known to be infected or colonized with A. baumannii (based on the presence of the organism in clinical samples such as urine, sputum, blood), the organism could be isolated from the skin in up to 70%. The rate of detection of A. baumannii in skin is very dependent on the method used to culture the skin, and does make interpretation and comparisons of different studies difficult. The bottom line seems to be that A. baumannii is a rare colonizer of non-hospitalized patients, but colonization (of skin, GIT or respiratory tract) becomes common the hospital setting, and probably commonest in ICU.

Environmental surfaces in hospitals can also be sources of A. baumannii, and a wide range of sources have been implicated in outbreaks, including ventilator tubing, ventilator tubing, suction catheters, humidifiers, containers of distilled water, urine collection jugs, multidose vials of medication, intravenous nutrition, moist bedding articles, inadequately sterilized reusable arterial pressure transducers, and computer keyboards. In addition, the ability of A. baumannii to survive for prolonged periods has been demonstrated repeatedly. One study demonstrated survival (in simulated hospital conditions) of 20 days, while others have isolated the organism from environmental sources over a week after discharge of the patient from the area. It is likely that the ability to form biofilms contributes to A. baumannii’s survival in the environment

Question 2: What infection control precautions should be used for A. baumannii?

Answer to Q2

The most common mode of transmission of A. baumannii is most probably via direct contact, on hands of health care workers. The most common source of the organism is thought to be the colonized or infected patient. However, the fact that the organism can remain viable in the environment for days may play a role in some outbreaks. There have been some reports describing possible airborne spread of A. baumannii. The evidence for this has primarily been the isolation of A. baumanniii on settle plates placed at variable distances (up to 11 feet / about 4 metres) from the index patient. However, most authorities still regard spread by direct contact as the most important mans of the organism spreading.

Given the above, implementation of strict contact precautions should be the first step in attempting to contain an outbreak. If possible, this should include isolation of the patient with the use of dedicated nursing staff. Unfortunately this is seldom, if ever, a realistic option in our setting. Due to the potential for environmental contamination, cleaning of the environment and medical equipment should be reviewed and prioritsed; antimicrobial stewardship should be enforced, and if appropriate, environmental samples can be taken to try to identify possible environmental source of the organism. As a last resort, units may need to be closed and thoroughly cleaned.

Question 3: What is the difference between cleaning, disinfection and sterilization?

Answer to Q3

Sterilisation is the process of destroying all pathogenic micro-organisms, including spores, on a device / object. Sterilisation is an absolute process – an item is either sterile or it is not (it cannot be partially sterile, in the same way that one cannot be partially pregnant or partially dead!)

Two of the commonly used methods of sterilization in South Africa (and worldwide) are steam sterilization and gas sterilization.

Autoclaving uses saturated steam under pressure, and is probably the most widely used sterilization method. Common steam sterilizing temperatures are 121°C or 132°C. Exposure times vary, depending on the type of material and the size of the load, but minimum exposure times are usually 30 mins at 121°C or 4 mins at 132°C. Steam sterilisation is cheap, effective and non-toxic. However, for obvious reasons it cannot be used on heat sensitive devices.

Ethylene oxide (ETO) is a colorless gas, which inactivates all microorganisms, including spores. However, spores are more resistant than other organisms. ETO is mainly used on instruments that cannot be steam sterilized. Its drawbacks include a lengthy cycle time, expense, and side effects. ETO can cause mucosal irritation and CNS depression when acute exposure occurs; chronic exposure has been associated with haematological abnormalities and malignancies. Staff safety when using ETO is thus an important consideration. Since many materials can absorb ETO, equipment must be adequately aerated after ETO sterilization prior to use on patients, for between 8 and 12 hours.

Other methods of low temperature sterilization include hydrogen peroxide gas plasma and peracteic acid. Both have their advantages and disadvantages. These, and other methods, are discussed comprehensively in the CDC’s 2008 “Guideline for Disinfection and Sterilisation in Healthcare Facilities”.

Disinfection involves the removal of most or all pathogenic organisms from a device / surface, with the exception of spores. While the main difference between disinfection and sterilization is thus the killing of spores, the process of disinfection may not necessarily kill all other organisms, and can be affected by the number of organisms, the exposure time, the temperature at which the process is performed. Disinfection is thus often divided into high level, intermediate level and low level disinfection.

High level disinfection destroys all microorganisms except spores, and is usually used for semi-critical items – those items that come into contact with mucous membranes and non-intact skin. This would include items such as anaesthetic equipment, some endoscopes and laryngoscopy blades. High level disinfection can be accomplished by heat (ie pasteurization; see below) or chemical disinfectants. Chemical disinfectants include peracetic acid with hydrogen peroxide, glutaraldehyde, and chlorine. For high level disinfection using chemical disinfectants, the time of exposure and concentration of the agent are vital. It is worth remembering that some chemical disinfectants can also act as sterilising agents, using appropriately higher concentrations, longer exposure times, higher temperatures, or combinations of these three variables.

Pasteurisation is not a sterilisation process. It was first described by Louis Pasteur as a method to prevent spoilage of wine (and later milk). In hospitals, devices disinfected by pasteurization are exposed to water at temperatures of at least 70°C for at least 30 minutes. Devices usually disinfected by pasteurization are ventilator tubing and anaesthetic circuits, which may not be amenable to disinfection by other methods, although some forms of chemical disinfection can also be used.

Intermediate and low level disinfection is reserved for non-critical patient items – items that come into contact with intact skin only. Intermediate level disinfection kills mycobacteria, while low level disinfection does not. Examples of these disinfectants are chlorine based products, phenols, quaternary ammonium compounds and alcohol.

Cleaning is the process of removing visible dirt and contamination (organic and inorganic matter) from a surface or an object. It usually involves use of water with detergents and/or enzymatic products. Cleaning is an absolute prerequisite before either disinfection or sterilization can occur. If there is visible soiling of an instrument, many disinfection or sterilization processes will be inefficient.

Outcome of the case:

In the last quarter of 2011, and more dramatically in the first quarter of 2012, the number of isolates of A. baumannii reduced dramatically (see Table 2). In addition, resistance to carbapenems (and other agents) also appears to be declining. Whether this is purely the result of the change to disposable ventilator circuits, or whether other interventions (for example the implementation of VAP bundles, and changing the processing of the emergency resuscitation equipment) played a role is hard to say.

Table 2: Number of A. baumannii bacteraemias, as well as number of isolates from respiratory tract samples, 2011-2012.

1: Resp tract isolates includes those from tracheal aspirate, broncho-alveolar lavage, sputum and induced sputum

Lessons learned:

Be persistent in trying to optimise all aspects of infection control in the hospital setting. Be open to exploring and reviewing as many avenues as possible.

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