Heat inactivation of Mycobacterium avium-Mycobacterium intracellulare complex organisms in aqueous suspension

Tenets of a holistic approach to drinking water-associated pathogen research, management, and communication

In recent years, drinking water-associated pathogens that can cause infections in immunocompromised or otherwise susceptible individuals (henceforth referred to as DWPI), sometimes referred to as opportunistic pathogens or opportunistic premise plumbing pathogens, have received considerable attention. DWPI research has largely been conducted by experts focusing on specific microorganisms or within silos of expertise. The resulting mitigation approaches optimized for a single microorganism may have unintended consequences and trade-offs for other DWPI or other interests (e.g., energy costs and conservation). For example, the ecological and epidemiological issues characteristic of Legionella pneumophila diverge from those relevant for Mycobacterium avium and other nontuberculous mycobacteria. Recent advances in understanding DWPI as part of a complex microbial ecosystem inhabiting drinking water systems continues to reveal additional challenges: namely, how can all microorganisms of concern be managed simultaneously? In order to protect public health, we must take a more holistic approach in all aspects of the field, including basic research, monitoring methods, risk-based mitigation techniques, and policy. A holistic approach will (i) target multiple microorganisms simultaneously, (ii) involve experts across several disciplines, and (iii) communicate results across disciplines and more broadly, proactively addressing source water-to-customer system management.

Molecular monitoring of disinfection efficacy using propidium monoazide in combination with quantitative PCR

One of the major drawbacks of DNA-based microbial diagnostics is its inability to discriminate between live and dead bacteria. Due to the persistence of DNA in the environment after cells have lost their viability, DNA-based assays cannot assess pathogenic risk since signals can originate from both live and dead cells. Presented here is a potential application of the novel chemical propidium monoazide (PMA), which results in the selective suppression of DNA detection from dead cells. PMA can only penetrate dead cells with permeabilized cell membranes. Upon intercalation into the DNA, covalent crosslinkage of PMA to DNA is achieved through light exposure. This modification prevents the DNA from being amplified by PCR. The method, in combination with quantitative PCR as a diagnostic tool, successfully monitored the disinfection efficacy of hypochlorite, benzalkonium and heat on several model pathogens. Threshold cycle numbers increased with increasing disinfection strength after PMA treatment of samples compared to non-PMA treated samples. With some disinfectant-specific differences, monitoring viability loss with membrane integrity as an indicator seemed to be more conservative than monitoring viability loss with plate counts. Loss of viability after short UV-exposure could not be monitored with PMA as UV light affects viability by inducing DNA damage without directly affecting membrane permeability.

Use of propidium monoazide for live/dead distinction in microbial ecology

One of the prerequisites of making ecological conclusions derived from genetic fingerprints is that bacterial community profiles reflect the live portion of the sample of interest. Propidium monoazide is a membrane-impermeant dye that selectively penetrates cells with compromised membranes, which can be considered dead. Once inside the cells, PMA intercalates into the DNA and can be covalently cross-linked to it, which strongly inhibits PCR amplification. By using PCR after PMA treatment, the analysis of bacterial communities can theoretically be limited to cells with intact cell membranes. Four experiments were performed to study the usefulness of PMA treatment of mixed bacterial communities comprising both intact and compromised cells in combination with end-point PCR by generating community profiles from the following samples: (i) defined mixtures of live and isopropanol-killed cells from pure cultures of random environmental isolates, (ii) wastewater treatment plant influent spiked with defined ratios of live and dead cells, (iii) selected environmental communities, and (iv) a water sediment sample exposed to increasing heat stress. Regions of 16S rRNA genes were PCR amplified from extracted genomic DNA, and PCR products were analyzed by using denaturing gradient gel electrophoresis (DGGE). Results from the first two experiments show that PMA treatment can be of value with end-point PCR by suppressing amplification of DNA from killed cells. The last two experiments suggest that PMA treatment can affect banding patterns in DGGE community profiles and their intensities, although the intrinsic limitations of end-point PCR have to be taken into consideration.

Colonization of hospital water systems by legionellae, mycobacteria and other heterotrophic bacteria potentially hazardous to risk group patients

Occurrences of legionellae and nontuberculous mycobacteria were followed in water systems of a tertiary care hospital where nosocomial infections due to the two genera had been verified. The aim was to examine whether their occurrence in the circulating hot water can be controlled by addition of a heat-shock unit in the circulation system, and by intensified cleaning of the tap and shower heads. One hot water system examined had an inbuilt heat-shock system causing a temporary increase of temperature to 80 degrees C, the other was an ordinary system (60 degrees C). The heat-shock unit decreased legionella colony counts in the circulating hot water (mean 35 cfu/l) compared to the ordinary system (mean 3.6 x 10(3) cfu/l). Mycobacteria constantly present in the incoming cold water (mean 260 cfu/l) were never isolated from the circulating hot water. Water sampled at peripheral sites such as taps and showers contained higher concentrations of legionellae, mycobacteria, and mesophilic and Gram-negative heterotrophs than the circulating waters. The shower water samples contained the highest bacterial loads. The results indicate the need to develop more efficient prevention methods than the ones presently used. Prevention of mycobacteria should also be extended to incoming cold water.

The genetics of nontuberculous mycobacterial infection

The molecular aetiology of familial susceptibility to disseminated mycobacterial disease, usually involving weakly pathogenic strains of mycobacteria, has now been elucidated in more than 30 families. Mutations have been identified in five genes in the interleukin-12-dependent interferon-gamma pathway, highlighting the importance of this pathway in human mycobacterial immunity. Knowledge derived from the study of these rare patients contributes to our understanding of the immune response to common mycobacterial pathogens such as Mycobacterium tuberculosis and Mycobacterium leprae, which remain major public health problems globally. This knowledge can be applied to the rational development of novel therapies and vaccines for these important mycobacterial diseases.

Copper and silver ions more effective against legionellae than against mycobacteria in a hospital warm water system

We studied the influence of electrolytically released copper and silver ions on the microbiological quality in a warm water system of a hospital. The concentration of nontuberculous mycobacteria was followed for three, and that of legionellae and other heterotrophic bacteria in the water for four years. The highest concentrations of copper and silver ions were 220 and 68 microg/l, respectively. Silver ion concentration of about 3 microg/l was sufficient to control the growth of legionellae in circulating warm water. The results showed that it is more difficult to eradicate legionellae from taps and showers: these points were colonized by a small number of legionellae after the metal ion concentrations were increased in the circulating water. A regular use of water eradicated legionellae from the shower. One tap was still used irregularly, and this may be a reason why it still contained small concentrations of legionellae also in the last years of the study. Mycobacteria were occasionally isolated from the circulating water and repeatedly from the shower, even when the metal concentrations were high. To control legionella bacteria in warm water systems, silver concentrations of only 3 microg/l are needed if all taps and showers of the system are regularly used. Such low copper and silver concentrations, however, are not efficient against nontuberculous mycobacteria or other heterotrophic bacteria.

Inactivation of Mycobacterium paratuberculosis in cows' milk at pasteurization temperatures

The thermal inactivation of 11 strains of Mycobacterium paratuberculosis at pasteurization temperatures was investigated. Cows' milk inoculated with M. paratuberculosis at two levels (10(7) and 10(4) CFU/ml) was pasteurized in the laboratory by (i) a standard holder method (63.5 degrees C for 30 min) and (ii) a high-temperature, short-time (HTST) method (71.7 degrees C for 15 s). Additional heating times of 5, 10, 15, 20, and 40 min at 63.5 degrees C were included to enable the construction of a thermal death curve for the organism. Viability after pasteurization was assessed by culture on Herrold's egg yolk medium containing mycobactin J (HEYM) and in BACTEC Middlebrook 12B radiometric medium supplemented with mycobactin J and sterile egg yolk emulsion. Confirmation of acid-fast survivors of pasteurization as viable M. paratuberculosis cells was achieved by subculture on HEYM to indicate viability coupled with PCR using M. paratuberculosis-specific 1S900 primers. When milk was initially inoculated with 10(6) to 10(7) CFU of M. paratuberculosis per ml, M. paratuberculosis cells were isolated from 27 of 28 (96%) and 29 of 34 (85%) pasteurized milk samples heat treated by the holder and HTST methods, respectively. Correspondingly, when 10(3) to 10(4) CFU of M. paratuberculosis per ml of milk were present before heat treatment, M. paratuberculosis cells were isolated from 14 of 28 (50%) and 19 of 33 (58%) pasteurized milk samples heat treated by the holder and HTST methods, respectively. The thermal death curve for M. paratuberculosis was concave in shape, exhibiting a rapid initial death rate followed by significant "tailing." Results indicate that when large numbers of M. paratuberculosis cells are present in milk, the organism may not be completely inactivated by heat treatments simulating holder and HTST pasteurization under laboratory conditions.

Persistent colonisation of potable water as a source of Mycobacterium avium infection in AIDS

The source of Mycobacterium avium infection in AIDS has not been identified and it is not known whether most patients with AIDS acquire the organism from recent infection or by reactivation of previous infection. As part of a prospective epidemiological study, we isolated multiple colonies of M avium from patients with AIDS and from potable water to which they had been exposed. All isolates were analysed with pulsed field gel electrophoresis (PFGE). As judged by PFGE, 29 (81%) of 36 patients were infected with one or more unique clinical strains of M avium. 7 patients (19%) were infected with three groups of common strains. Group 1 included 3 patients who lived in separate rural areas and had no common exposures apart from treatment at hospital A. The same strain was isolated repeatedly during 41 months from a recirculating hot water system at hospital A; residential water cultures were negative. Group 2 included 2 patients with no common exposures apart from treatment at hospital B; the same strain was isolated repeatedly over a period of 24 months from a recirculating hot water system at hospital B. Patients in groups 1 and 2 had numerous possible exposures to hospital hot water. Group 3 included 2 patients treated at the same methadone treatment facility. In an institution the hot water system may be persistently colonised with a particular strain of M avium. HIV-infected patients exposed to these water sources can develop disseminated M avium infection.

Heat inactivation of in vivo- and in vitro-grown mycobacteria in meat products

Heat inactivation of mycobacteria from lesions and from culture was compared in meat products. In vivo-grown organisms were more easily heat inactivated than were in vitro-grown organisms.

Inactivation of Mycobacterium bovis in meat products

The time-temperature combinations necessary to destroy Mycobacterium bovis in meat products were determined. In any given time, M. bovis was destroyed at temperatures 6 to 7 degrees C (ca. 12 degrees F) lower than those necessary for destruction of members of the Mycobacterium avium-Mycobacterium intracellulare complex. Hence, any processing heat adequate to kill M. avium-M. intracellulare-complex organisms will provide a very large safety factor with respect to M. bovis. Benzalkonium chloride treatment of wiener specimens for cultural examination effectively destroyed the normal flora of wiener emulsion without reducing the numbers of M. bovis. Treatment with a phenolic disinfectant followed by formaldehyde vapor was effective in disinfecting equipment contaminated with meat emulsion containing M. bovis.

Heat inactivation of Mycobacterium avium-Mycobacterium intracellulare complex organisms in meat products

Wieners and sausages were prepared which contained the most heat-tolerant representative of the Mycobacterium avium-Mycobacterium intracellulare complex we were able to obtain. They also were prepared with infected tissues obtained from tuberculous swine. Processing conditions were as varied as possible. Neither incorporation of sodium nitrite in the emulsion nor presence of smoke during processing altered the heat susceptibility of the organisms. Substantial killing of the organisms occurred as wieners reached the upper processing temperatures, but hot oil or radiant heating of the "precooked" sausages allowed very short times within the killing range; hence, higher peak internal temperatures were necessary. The lethalities for these organisms of reaching and maintaining various processing temperatures are given.