Mycobacterium chelonae– a growing problem in soft tissue infection

Mycobacterium chelonae is a fast growing mycobacterium isolated from environmental, nonhuman animal and human sources. Now classified as part of the M. fortuitum complex, it is closely related to M. abscessus and M. immunogenum, sometimes known as the M. chelonae/abscessus group [1]. These are nonpigmenting organisms that achieve growth within 7 days on normal laboratory media. Members of the M. fortuitum complex, along with M. ulcerans and M. marinum, are often associated with soft tissue infection. In recent years, M. chelonae has been associated with a range of human infection, usually localised, but occasionally giving rise to haematogenously spread infections [2]. It is also known as an agent associated with cutaneous and soft tissue infection such as abscesses, cellulitis and subcutaneous infection associated with introduction of cannulae and synthetic materials. It has also been associated with infection of the cornea and appendages of the eye such as the lacrimal sac, as well as implanted ocular materials [3]. Although cases are often sporadic it may lead to clustered cases of infections, which may be associated with exposure to common environmental sources of the organisms such as contaminated water supplies or injected materials. M. chelonae was originally classified as a fast growing member of the nontuberculous mycobacteria, but with the modern genetic classification system it appears to be closely related to organisms such as M. abscessus. It can be identified in culture, producing visible growth on agar within a week although it may grow best on primary isolation at 28°C. However, increasingly it is more accurate and faster to use RNA probes to identify this organism to species level [4]. Likewise probes can also be employed on either pathological or human samples using different PCR-based techniques. In the skin M. chelonae is seen as a cause of infections such as abscesses and increasingly in areas that have been subject to cosmetic procedures [5,6] such as injection sites of botulinum toxin (Botox) [7] or liposuction [8] or under skin flaps [9]. The use of surgical procedures affecting the skin has long been known as a potential source of infection so that laparascopic sites [10] and the insertion sites of peritoneal dialysis catheters [11] or Hickman lines [12] are also potential sources. Interestingly, concurrent disease or clinical management processes that lead to immunosuppression are usually associated with more severe and sometimes disseminated disease [2,13,14]. Although resistance is thought to depend on Th1 mediated pathways, severe infection is also seen in the severely neutropenic patient; infection has also been associated with the therapeutic use of biologics that have specific T-cell directed activity, such as adalimumab [15]. It is not clear though how and why this infection is initiated. M. chelonae can be isolated from a wide range of environmental sources such as tap water and other sources of water including water tanks and industrial sources [16]. In the context of the wider community, this organism has also been isolated from other sources such as city drinking water supplies [17,18] and it has also been found on medical instruments such as bronchoscopes. M. chelonae has been isolated from both natural and healthcare environments, for example, foot baths in clinics [19]. This can give rise to pseudo-outbreaks of infection such as occurred when a cleaning water reservoir used in bronchoscope cleaning was found to contain M. chelonae[20]. On surfaces it can form biofilms. This contrasts with M. fortuitum infections where the majority of patients have often received a soft tissue injury associated with trauma such as a road traffic accident. There have been cases associated with the use of contaminated footbaths in a beauty salon where some, but not all, of the individuals had also received minor traumatic skin procedures such as epilation [19]. Given the likely risk of environmental exposure rate of patients to M. chelonae, it is interesting to consider what allows the organism to gain invasive advantage. It may be that the development of either specific resistance mechanisms or cell wall changes is responsible for survival in the conditions associated with human infection. In normal circumstances, M chelonae has a well-structured cell wall with a mycolic acid residue bilayer, which is relatively impermeable [21]. The major glycolipid components, lipomannans or lipoarabinomannans, are very different in M. chelonae compared with other mycobacteria studied. There are few known correlates of pathogenicity in the cell wall structure of this organism, unlike others such as M. tuberculosis, but for instance, a glutaraldehyde resistant strain showed correlation with reduced arabinogalactan in the cell wall, associated with increased hydrophobicity, and this may correlate with reduced penetration of bacterial cells by molecules such as glutaraldehyde. Lipomannan extracted from M. chelonae can induce tumour necrosis factor (TNF)-α and interleukin (IL)-8 secretion in the human macrophage-like THP-1 cell line in a dose-dependent manner. This can be abrogated by deacylation, showing the importance of the lipid component of these molecules in ensuring signalling capacity. However, with lipoarabinomannans there is also an inhibitory effect as the arabinan domain of lipoarabinomannan from M. chelonae prevents recognition of this molecule by lectins such as concanavalin A [22]. The proposed mechanism is through stearic hindrance with binding sites. The formation of biofilms may also aid survival and spread. An early experiment using polyvinylchrolide tubes filled with distilled water and then inoculated with bacteria showed that M. chelonae (with other nonmycobacterial organisms) survived the addition of germicides such as iodophor disinfectant, phenolic detergent, quaternary ammonium compound, 2% formaldehyde and 2% glutaraldehyde [23]. Scrapings taken from the tube after the experiment showed that the bacteria were embedded in extracellular material, suggesting that the process of growth had resulted in the formation of a protective barrier. It is not clear how the structure of any biofilm in which M. chelonae is present is shaped. Although different biofilm structures have been proposed, one that accomodates greater growth at the upper surface compared with the inner attachment zone – a stalk-like model – with penetrating microchannels piercing the matrix [24] is one possible model adopted under environmental conditions. In another skin invader, M. ulcerans, the biofilms are very distinctive. There is a large amount of extracellular matrix that is both protein and carbohydrate rich, but it also contains vesicle-like structures that contain mycolactone, which is an important virulence determinant of this species. Inoculation of M. ulcerans into mice together with mycolactone results in a more extensive infection [25]. Hence, the presence of an accompanying virulence determinant in naturally occurring biofilm may have a direct effect on subsequent infection. There is little evidence that similar adaptations affect M. chelonae infections and by contrast M. chelonae and M. fortuitum biofilms mainly consist of tightly clustered cells with a smaller extracellular matrix; M. chelonae does, however, produce some extracellular material when present in a biofilm [23]. At this stage, though, we cannot argue that organisms such as M. chelonae carry with them potential extracellular virulence factors present in an aquatic biofilm. In addition, there other virulence factors that appear to influence the course of infection. For instance, with M. chelonae the presence of a gene, MtrA, appears to be important for invasion of cells at 37°C or cross-species infection. This may not hinder its replication in the environment but it does appear to be necessary for replication in animal cells [26]. Our knowledge of Mycobacterium chelonae as a cause of soft tissue infection is growing. However, as with many organisms that are present in the environment and where the potential for human exposure is large, the risk of infection resulting from contact or inoculation during or after an invasive procedure remains largely unknown. However, even an organism such as this, which is often regarded as having low pathogenic potential, has a number of virulence determinants that are likely to play key roles in infection.