Surviving the acid test: responses of gram-positive bacteria to low pH

Gram-positive bacteria possess a myriad of acid resistance systems that can help them to overcome the challenge posed by different acidic environments. In this review the most common mechanisms are described: i.e., the use of proton pumps, the protection or repair of macromolecules, cell membrane changes, production of alkali, induction of pathways by transcriptional regulators, alteration of metabolism, and the role of cell density and cell signaling. We also discuss the responses of Listeria monocytogenes, Rhodococcus, Mycobacterium, Clostridium perfringens, Staphylococcus aureus, Bacillus cereus, oral streptococci, and lactic acid bacteria to acidic environments and outline ways in which this knowledge has been or may be used to either aid or prevent bacterial survival in low-pH environments.

M. tuberculosis and M. leprae Translocate from the Phagolysosome to the Cytosol in Myeloid Cells

M. tuberculosis and M. leprae are considered to be prototypical intracellular pathogens that have evolved strategies to enable growth in the intracellular phagosomes. In contrast, we show that lysosomes rapidly fuse with the virulent M. tuberculosis- and M. leprae-containing phagosomes of human monocyte-derived dendritic cells and macrophages. After 2 days, M. tuberculosis progressively translocates from phagolysosomes into the cytosol in nonapoptotic cells. Cytosolic entry is also observed for M. leprae but not for vaccine strains such as M. bovis BCG or in heat-killed mycobacteria and is dependent upon secretion of the mycobacterial gene products CFP-10 and ESAT-6. The cytosolic bacterial localization and replication are pathogenic features of virulent mycobacteria, causing significant cell death within a week. This may also reveal a mechanism for MHC-based antigen presentation that is lacking in current vaccine strains.

TNF dually mediates resistance and susceptibility to mycobacteria via mitochondrial reactive oxygen species

Tumor necrosis factor (TNF) constitutes a critical host defense against tuberculosis, but its excess is also implicated in tuberculosis pathogenesis in zebrafish and humans. Using the zebrafish, we elucidate the pathways by which TNF mediates tuberculosis pathogenesis. TNF excess induces mitochondrial reactive oxygen species (ROS) in infected macrophages through RIP1-RIP3-dependent pathways. While initially increasing macrophage microbicidal activity, ROS rapidly induce programmed necrosis (necroptosis) and release mycobacteria into the growth-permissive extracellular milieu. TNF-induced necroptosis occurs through two pathways: modulation of mitochondrial cyclophilin D, implicated in mitochondrial permeability transition pore formation, and acid sphingomyelinase-mediated ceramide production. Combined genetic blockade of cyclophilin D and acid sphingomyelinase renders the high TNF state hyperresistant by preventing macrophage necrosis while preserving increased microbicidal activity. Similarly, the cyclophilin D-inhibiting drug alisporivir and the acid sphingomyelinase-inactivating drug, desipramine, synergize to reverse susceptibility, suggesting the therapeutic potential of these orally active drugs against tuberculosis and possibly other TNF-mediated diseases.

Long-lasting effects of BCG vaccination on both heterologous Th1/Th17 responses and innate trained immunity

We have recently shown that BCG (Bacillus Calmette-Guérin) vaccination in healthy volunteers induces epigenetic reprogramming of monocytes, leading to increased cytokine production in response to nonrelated pathogens for up to 3 months after vaccination. This phenomenon was named 'trained immunity'. In the present study we assessed whether BCG was able to induce long-lasting effects on both trained immunity and heterologous T helper 1 (Th1) and Th17 immune responses 1 year after vaccination. The production of TNFα and IL-1β to mycobacteria or unrelated pathogens was higher after 2 weeks and 3 months postvaccination, but these effects were less pronounced 1 year after vaccination. However, monocytes recovered 1 year after vaccination had an increased expression of pattern recognition receptors such as CD14, Toll-like receptor 4 (TLR4) and mannose receptor, and this correlated with an increase in proinflammatory cytokine production after stimulation with the TLR4 ligand lipopolysaccharide. The heterologous production of Th1 (IFN-γ) and Th17 (IL-17 and IL-22) immune responses to nonmycobacterial stimulation remained strongly elevated even 1 year after BCG vaccination. In conclusion, BCG induces sustained changes in the immune system associated with a nonspecific response to infections both at the level of innate trained immunity and at the level of heterologous Th1/Th17 responses.

The secret lives of the pathogenic mycobacteria

Pathogenic mycobacteria, including the causative agents of tuberculosis and leprosy, are responsible for considerable morbidity and mortality worldwide. A hallmark of these pathogens is their tendency to establish chronic infections that produce similar pathologies in a variety of hosts. During infection, mycobacteria reside in macrophages and induce the formation of granulomas, organized immune complexes of differentiated macrophages, lymphocytes, and other cells. This review summarizes our understanding of Mycobacterium-host cell interactions, the bacterial-granuloma interface, and mechanisms of bacterial virulence and persistence. In addition, we highlight current controversies and unanswered questions in these areas.

rpoB gene sequence-based characterization of emerging non-tuberculous mycobacteria with descriptions of Mycobacterium bolletii sp. nov., Mycobacterium phocaicum sp. nov. and Mycobacterium aubagnense sp. nov

Over the past 10 years, 16S rRNA gene sequencing has contributed to the establishment of more than 45 novel species of non-tuberculous mycobacteria and to the description of emerging mycobacterial infections. Cumulative experience has indicated that this molecular tool underestimates the diversity of this group and does not distinguish between all recognized mycobacterial taxa. In order to improve the recognition of emerging rapidly growing mycobacteria (RGM), rpoB gene sequencing has been developed. Our previous studies have shown that an RGM isolate is a member of a novel species if it exhibits >3 % sequence divergence in the rpoB gene from the type strains of established species. When applied to a collection of 59 clinical RGM isolates, rpoB gene sequencing revealed nine novel isolates (15.3 %) whereas only two isolates (3.4 %) were deemed to be novel by conventional 16S rRNA gene sequence analysis. A polyphasic approach, including biochemical tests, antimicrobial susceptibility analyses, hsp65, sodA and recA gene sequence analysis, DNA G+C content determination and cell-wall fatty acid composition analysis, supported the evidence that these nine isolates represent three novel species. Whereas Mycobacterium phocaicum sp. nov. (type strain N4T = CIP 108542T = CCUG 50185T) and Mycobacterium aubagnense sp. nov. (type strain U8T = CIP 108543T = CCUG 50186T; Mycobacterium mucogenicum group) were susceptible to most antibiotics, Mycobacterium bolletii sp. nov. (type strain BD(T) = CIP 108541T = CCUG 50184T; Mycobacterium chelonae-abscessus group) was resistant to the quinolones, tetracycline, macrolides and imipenem. Only M. bolletii was resistant to clarithromycin. These data illustrate that rpoB gene sequence-based identification is a powerful tool to characterize emerging RGM and mycobacterial infections and provides valuable help in differentiating RGM at both the intra- and interspecies level, thus contributing to a faster and more efficient diagnosis and epidemiological follow-up.

Isolation of adherent polycyclic aromatic hydrocarbon (PAH)-degrading bacteria using PAH-sorbing carriers

Two different procedures were compared to isolate polycyclic aromatic hydrocarbon (PAH)-utilizing bacteria from PAH-contaminated soil and sludge samples, i.e., (i) shaken enrichment cultures in liquid mineral medium in which PAHs were supplied as crystals and (ii) a new method in which PAH degraders were enriched on and recovered from hydrophobic membranes containing sorbed PAHs. Both techniques were successful, but selected from the same source different bacterial strains able to grow on PAHs as the sole source of carbon and energy. The liquid enrichment mainly selected for Sphingomonas spp., whereas the membrane method exclusively led to the selection of Mycobacterium spp. Furthermore, in separate membrane enrichment set-ups with different membrane types, three repetitive extragenic palindromic PCR-related Mycobacterium strains were recovered. The new Mycobacterium isolates were strongly hydrophobic and displayed the capacity to adhere strongly to different surfaces. One strain, Mycobacterium sp. LB501T, displayed an unusual combination of high adhesion efficiency and an extremely high negative charge. This strain may represent a new bacterial species as suggested by 16S rRNA gene sequence analysis. These results indicate that the provision of hydrophobic sorbents containing sorbed PAHs in the enrichment procedure discriminated in favor of certain bacterial characteristics. The new isolation method is appropriate to select for adherent PAH-degrading bacteria, which might be useful to biodegrade sorbed PAHs in soils and sludge.

Survival of mycobacteria in macrophages is mediated by coronin 1-dependent activation of calcineurin

Pathogenic mycobacteria survive within macrophages by avoiding lysosomal delivery, instead residing in mycobacterial phagosomes. Upon infection, the leukocyte-specific protein coronin 1 is actively recruited to mycobacterial phagosomes, where it blocks lysosomal delivery by an unknown mechanism. Analysis of macrophages from coronin 1-deficient mice showed that coronin 1 is dispensable for F-actin-dependent processes such as phagocytosis, motility, and membrane ruffling. However, upon mycobacterial infection, coronin 1 was required for activation of the Ca(2+)-dependent phosphatase calcineurin, thereby blocking lysosomal delivery of mycobacteria. In the absence of coronin 1, calcineurin activity did not occur, resulting in lysosomal delivery and killing of mycobacteria. Furthermore, blocking calcineurin activation with cyclosporin A or FK506 led to lysosomal delivery and intracellular mycobacterial killing. These results demonstrate a role for coronin 1 in activating Ca(2+) dependent signaling processes in macrophages and reveal a function for calcineurin in the regulation of phagosome-lysosome fusion upon mycobacterial infection.

Role of M cells and macrophages in the entrance of Mycobacterium paratuberculosis into domes of ileal Peyer's patches in calves

Ligated ileal loops of calves were inoculated with live and heat-killed Mycobacterium paratuberculosis and were examined by light and electron microscopy. At 5 hours after inoculation, acid-fast bacilli were in subepithelial macrophages, but not in M cells covering domes. At 20 hours, more than 50 acid-fast bacilli per cross section were in subepithelial macrophages in domes. Both living and heat-killed bacilli passed into domes. Addition of anti-M. paratuberculosis bovine serum to the inoculum enhanced entry of bacteria into domes. By electron microscopy, intact bacilli with electron-transparent zones (peribacillary spaces) were in the supranuclear cytoplasm of M cells at 20 hours. M cells also contained vacuoles, including electron-dense material interpreted as degraded bacilli. Subepithelial and intraepithelial macrophages contained bacilli and degraded bacterial material in phagosomes. These results suggest that calf ileal M cells take up bacilli, and that subepithelial and intraepithelial macrophages secondarily accept bacilli or bacterial debris which are expelled from M cells.

Applications for green fluorescent protein (GFP) in the study of host-pathogen interactions

The green fluorescent protein (GFP) from Aequorea victoria is a novel fluorescent marker that has potential use in the study of bacterial pathogenicity. To explore some of the potential applications of GFP to the study of host-parasite interactions, we constructed two GFP expression vectors suitable for different facultative intracellular bacterial pathogens. The first expression vector was tested in the enteric pathogens, Salmonella typhimurium and Yersinia pseudotuberculosis, and the second vector tested in Mycobacterium marinum (Mm). Both expression vectors were found to be stable and to direct high levels of GFP synthesis. Standard epifluorescence microscopy was used to detect all three bacterial pathogenic species during the early and late stages of infection of live mammalian cells. Mm expressing gfp was also visualized in infected animal tissues. gfp expression did not adversely affect bacterial survival, nor did it compromise entry into mammalian cells or their survival within macrophages. In addition, all three gfp-expressing bacterial pathogens could be detected and sorted in a flow cytometer, either alone or in association with epithelial cells or macrophages. Therefore, GFP not only provides a convenient tool to image pathogenic bacteria, but allows the quantitative measurement of bacterial association with mammalian cells.

Intracellular signalling cascades regulating innate immune responses to Mycobacteria: branching out from Toll-like receptors

Toll-like receptors (TLRs) recognize Mycobacterium tuberculosis (Mtb) or Mtb components and initiate mononuclear phagocyte responses that influence both innate and adaptive immunity. Recent studies have revealed the intracellular signalling cascades involved in the TLR-initiated immune response to mycobacterial infection. Although both TLR2 and TLR4 have been implicated in host interactions with Mtb, the relationship between specific mycobacterial molecules and various signal transduction pathways is not well understood. This review will discuss recent studies indicating critical roles for mycobacteria and mycobacterial components in regulation of mitogen-activated protein kinases and related signal transduction pathways that govern the outcome of infection and antibacterial defence. To better understand the roles of infection-induced signalling cascades in molecular pathogenesis, future studies are needed to clarify mechanisms that integrate the multiple signalling pathways that are activated by engagement of TLRs by both individual mycobacterial molecules and whole mycobacteria. These efforts will allow for the development of novel diagnostic and therapeutic modalities for tuberculosis that targets the intracellular signalling pathways permitting the replication of this nefarious pathogen.

Mycolic acid structure determines the fluidity of the mycobacterial cell wall

The low permeability of the mycobacterial cell wall is thought to contribute to the well known resistance of mycobacteria to antibiotics and chemotherapeutic agents. We have used differential scanning calorimetry to demonstrate that the high temperature phase transition observed in purified cell walls, usually in the 60-70 degrees C range, suggestive of a lipid environment of extremely low fluidity, can also be observed in whole organisms and in cell walls from which much of the free lipids was removed by extraction with Triton X-114. A survey of seven mycobacterial species demonstrated that this high temperature transition was a general property of these organisms. Cell walls isolated from two Corynebacterium species, which contain much shorter corynemycolic acids, displayed a much lower temperature transition, suggesting that the transition temperature was directly correlated to the length of mycolic acid. Methyl esters of mycolic acids were found to have a phase transition temperature that was linearly related to the amount of trans-mycolate. Both Mycobacterium avium and M. smegmatis responded to increasing growth temperature by increasing the proportion of trans-mycolate and displaying a correspondingly higher melting temperature. Whole cells of M. smegmatis grown at higher temperature allowed a less rapid influx of two lipophilic agents, norfloxacin and chenodeoxycholate. These results provide strong evidence that the nature of mycolic acid plays a crucial role in determining the fluidity and permeability of mycobacterial cell wall.

The cell surface receptor DC-SIGN discriminates between Mycobacterium species through selective recognition of the mannose caps on lipoarabinomannan

Interactions between dendritic cells (DCs) and Mycobacterium tuberculosis, the etiological agent of tuberculosis, most likely play a key role in anti-mycobacterial immunity. We have recently shown that M. tuberculosis binds to and infects DCs through ligation of the DC-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) and that M. tuberculosis mannose-capped lipoarabinomannan (ManLAM) inhibits binding of the bacilli to the lectin, suggesting that ManLAM might be a key DC-SIGN ligand. In the present study, we investigated the molecular basis of DC-SIGN ligation by LAM. Contrary to what was found for slow growing mycobacteria, such as M. tuberculosis and the vaccine strain Mycobacterium bovis bacillus Calmette-Guérin, our data demonstrate that the fast growing saprophytic species Mycobacterium smegmatis hardly binds to DC-SIGN. Consistent with the former finding, we show that M. smegmatis-derived lipoarabinomannan, which is capped by phosphoinositide residues (PILAM), exhibits a limited ability to inhibit M. tuberculosis binding to DC-SIGN. Moreover, using enzymatically demannosylated and chemically deacylated ManLAM molecules, we demonstrate that both the acyl chains on the ManLAM mannosylphosphatidylinositol anchor and the mannooligosaccharide caps play a critical role in DC-SIGN-ManLAM interaction. Finally, we report that DC-SIGN binds poorly to the PILAM and uncapped AraLAM-containing species Mycobacterium fortuitum and Mycobacterium chelonae, respectively. Interestingly, smooth colony-forming Mycobacterium avium, in which ManLAM is capped with single mannose residues, was also poorly recognized by the lectin. Altogether, our results provide molecular insight into the mechanisms of mycobacteria-DC-SIGN interaction, and suggest that DC-SIGN may act as a pattern recognition receptor and discriminate between Mycobacterium species through selective recognition of the mannose caps on LAM molecules.

A specific secretion system mediates PPE41 transport in pathogenic mycobacteria

Mycobacterial genomes contain two unique gene families, the so-called PE and PPE gene families, which are highly expanded in the pathogenic members of this genus. Here we report that one of the PPE proteins, i.e. PPE41, is secreted by pathogenic mycobacteria, both in culture and in infected macrophages. As PPE41 lacks a signal sequence a dedicated secretion system must be involved. A single gene was identified in Mycobacterium marinum that showed strongly reduced PPE41 secretion. This gene was located in a gene cluster whose predicted proteins encode components of an ESAT-6-like secretion system. This cluster, designated ESX-5, is conserved in various pathogenic mycobacteria, but not in the saprophytic species Mycobacterium smegmatis. Therefore, different regions of this cluster were introduced in M. smegmatis. Only introduction of the complete ESX-5 locus resulted in efficient secretion of heterologously expressed PPE41. This PPE secretion system is also involved in the virulence of pathogenic mycobacteria, as the ESX-5 mutant of M. marinum was affected in spreading to uninfected macrophages.

Evidence for inhibition of fusion of lysosomal and prelysosomal compartments with phagosomes in macrophages infected with pathogenic Mycobacterium avium

Bone marrow-derived cultured macrophages were infected with the pathogenic organism Mycobacterium avium. Immediately after infection and at 1 to 28 days later, cells either were stained for acid phosphatase activity or given horseradish peroxidase, which served as a pinocytotic marker. With the former, fusions between phagosomes and lysosomes exclusively were assessed; with the latter, those between phagosomes and both pinosomes and lysosomes were determined. As a control, similar experiments were undertaken by infecting macrophages with gamma ray-killed M. avium and the nonpathogenic live organisms Mycobacterium aurum and Bacillus subtilis. After infection with live M. avium, fusions between phagosomes and acid phosphatase-positive vesicles (lysosomes) were inhibited. The same inhibition was observed whether phagosomes contained damaged or structurally intact (presumed to be live) bacteria, except for the early time points. This inhibition was, however, partial, suggesting that some of the live bacteria are resistant to the hydrolytic enzymes of the phagolysosomal environment. Fusions between horseradish peroxidase-positive vesicles (pinosomes and lysosomes) and phagosomes depended upon the morphological state of the bacteria. Damaged bacteria did not inhibit fusions, whereas with intact bacteria, a partial inhibition which increased with time was observed. The two types of experiment suggest that viable M. avium can impair phagosome-pinosome fusions.

Green fluorescent protein as a marker for gene expression and cell biology of mycobacterial interactions with macrophages

The green fluorescent protein (GFP) of the jellyfish Aequorea victoria offers certain advantages over other bioluminescence systems because no exogenously added substrate or co-factors are necessary, and fluorescence can be elicited by irradiation with blue light without exposing the cells producing GFP to invasive treatments. A mycobacterial shuttle-plasmid vector carrying gfp cDNA was constructed and used to generate transcriptional fusions with promoters of interest and to examine their expression in Mycobacterium smegmatis and Mycobacterium bovis BCG grown in macrophages or on laboratory media. The promoters studied were: (i) ahpC from Mycoosis and Mycobacterium leprae, a gene encoding alkyl hydroperoxide reductase which, along with the divergently transcribed regulator oxyR, are homologues of corresponding stress-response systems in enteric bacteria and play a role in isoniazid sensitivity; (ii) mtrA, an M. tuberculosis response regulator belonging to the superfamily of bacterial two-component signal-transduction systems; (iii) hsp60, a previously characterized heat-shock gene from M. bovis; and (iv) tbprc3, a newly isolated promoter from M. tuberculosis. Expression of these promoters in mycobacteria was analysed using epifluorescence microscopy, laser scanning confocal microscopy, fluorescence spectroscopy, and flow cytometry. These approaches permitted assessment of fluorescence prior to and after macrophage infection, and analyses of promoter expression in individual mycobacteria and its distribution within populations of bacterial cells. Bacteria expressing GFP from a strong promoter could be separated by fluorescence-activated cell sorting from cells harbouring the vector used to construct the fusion. In addition, the stable expression of mtrA-gfp fusion in M. bovis BCG facilitated localization and isolation of phagocytic vesicles containing mycobacteria. The experiments presented here suggest that GFP will be a useful tool for analysis of mycobacterial gene expression and a convenient cell biology marker to study mycobacterial interactions with macrophages.

Degradation of aflatoxin B1 by cell-free extracts of Rhodococcus erythropolis and Mycobacterium fluoranthenivorans sp. nov. DSM44556T

Biological degradation of aflatoxin B(1) (AFB(1)) by Rhodococcus erythropolis was examined in liquid cultures and in cell-free extracts. Dramatic reduction of AFB(1) was observed during incubation in the presence of R. erythropolis cells (17% residual AFB(1) after 48 h and only 3-6% residual AFB(1) after 72 h). Cell-free extracts of four bacterial strains, R. erythropolis DSM 14,303, Nocardia corynebacterioides DSM 12,676, N. corynebacterioides DSM 20,151, and Mycobacterium fluoranthenivorans sp. nov. DSM 44,556(T) were produced by disrupting cells in a French pressure cell. The ability of crude cell-free extracts to degrade AFB(1) was studied under different incubation conditions. Aflatoxin B(1) was effectively degraded by cell free extracts of all four bacterial strains. N. corynebacterioides DSM 12,676 (formerly erroneously classified as Flavobacterium aurantiacum) showed the lowest degradation ability (60%) after 24 h, while >90% degradation was observed with N. corynebacterioides DSM 20,151 over the same time. R. erythropolis and M. fluoranthenivorans sp. nov. DSM 44,556(T) have shown more than 90% degradation of AFB(1) within 4 h at 30 degrees C, whilst after 8 h AFB(1) was practicably not detectable. The high degradation rate and wide temperature range for degradation by R. erythropolis DSM 14,303 and M. fluoranthenivorans sp. nov. DSM 44,556(T) indicate potential for application in food and feed processing.

Disparate responses to oxidative stress in saprophytic and pathogenic mycobacteria

To persist in macrophages and in granulomatous caseous lesions, pathogenic mycobacteria must be equipped to withstand the action of toxic oxygen metabolites. In Gram-negative bacteria, the OxyR protein is a critical component of the oxidative stress response. OxyR is both a sensor of reactive oxygen species and a transcriptional activator, inducing expression of detoxifying enzymes such as catalase/hydroperoxidase and alkyl hydroperoxidase. We have characterized the responses of various mycobacteria to hydrogen peroxide both phenotypically and at the levels of gene and protein expression. Only the saprophytic Mycobacterium smegmatis induced a protective oxidative stress response analogous to the OxyR response of Gram-negative bacteria. Under similar conditions, the pathogenic mycobacteria exhibited a limited, nonprotective response, which in the case of Mycobacterium tuberculosis was restricted to induction of a single protein, KatG. We have also isolated DNA sequences homologous to oxyR and ahpC from M. tuberculosis and Mycobacterium avium. While the M. avium oxyR appears intact, the oxyR homologue of M. tuberculosis contains numerous deletions and frameshifts and is probably nonfunctional. Apparently the response of pathogenic mycobacteria to oxidative stress differs significantly from the inducible OxyR response of other bacteria.

The Trojan horse: survival tactics of pathogenic mycobacteria in macrophages

Mycobacterium tuberculosis, the causative agent of tuberculosis, has infected billions of people worldwide. A key to the success of M. tuberculosis and related pathogenic mycobacteria lies in their ability to persist within the hostile environment of the host macrophage. After internalization by macrophages, most microbes are rapidly transported to lysosomes in which they are destroyed. By contrast, pathogenic mycobacteria prevent fusion of phagosomes with lysosomes, thereby surviving intracellularly. Recent progress in understanding the molecular biology of host-mycobacteria interactions is providing insights into these survival tactics.

Phagocytosis: latex leads the way

Phagocytosis is the process that cells have evolved to internalise large particles such as mineral debris, which they store, or apoptotic cells and pathogens, which they have the capacity to kill and degrade. However, several important pathogens can suppress these killing functions and survive and multiply within phagosomes, causing disease. Recent advances in phagosome biology have been made possible largely by a model system that uses inert latex beads. The ability to purify latex bead-containing phagosomes has opened the door to allow comprehensive biochemical analyses and functional assays to study the molecular mechanisms governing phagosome function. These approaches have led to unique insights directly relevant for the understanding of the biology of intracellular pathogens and the ways by which they subvert their hosts.

Mycobacterium wolinskyi sp. nov. and Mycobacterium goodii sp. nov., two new rapidly growing species related to Mycobacterium smegmatis and associated with human wound infections: a cooperative study from the International Working Group on Mycobacterial Taxonomy

Previous investigations demonstrated three taxonomic groups among 22 clinical isolates of Mycobacterium smegmatis. These studies were expanded to 71 clinical isolates, of which 35 (49%) (group 1) were identical to five ATCC reference strains including the type strain ATCC 19420T. Twenty-eight isolates (39%) were group 2, and eight isolates (11%) were group 3. Isolates of groups 2 and 3 were most often associated with post-traumatic or post-surgical wound infections including osteomyelitis, were susceptible to sulfamethoxazole, amikacin, imipenem and the tetracyclines, variably resistant to clarithromycin, and susceptible (group 1), intermediately resistant (group 2) or resistant (group 3) to tobramycin. The three groups were similar by routine biochemical and growth characteristics, but had different mycolic acid dimethoxy-4-coumarinylmethyl ester elution patterns by HPLC and different PCR-restriction enzyme patterns of a 439 bp fragment of the hsp-65 gene. Group 3 isolates differed from group 1 by 18 bp by 16S rRNA sequencing and exhibited < 25% homology by DNA-DNA hybridization, being most closely related to Mycobacterium mageritense. The 16S rRNA of group 1 and group 2 isolates differed by only 3 bp, but by DNA-DNA hybridization they exhibited only 40% homology. The following names are proposed: Mycobacterium goodii sp. nov. for group 2 isolates (type strain ATCC 700504T = MO69T), Mycobacterium wolinskyi sp. nov. for group 3 isolates (type strain ATCC 700010T = MO739T) and Mycobacterium smegmatis sensu stricto for group 1 isolates.

Oxygen depletion induced dormancy in Mycobacterium smegmatis

We report here that the physiological behaviour of the fast growing saprophytic Mycobacterium smegmatis under in vitro oxygen-depletion and reactivation conditions is strikingly similar to the characteristics shown by the slow growing pathogenic M. tuberculosis. M. smegmatis died rapidly when shifted abruptly from aerobic to anaerobic conditions. In contrast to the lethal shock of abrupt oxygen depletion, the slow depletion through a self generated oxygen gradient permitted an adaptation to a persistent state which showed increased resistance against the bactericidal effects of anaerobiosis. The anaerobic persistent culture did not synthesise DNA and showed synchronised division upon reactivation in oxygen rich medium, indicating that the persistent bacilli are uniformly arrested at a defined stage of the cell cycle. Upon reactivation the persistent culture started synthesising DNA only after the first cell division, suggesting that the persistent cells contain two chromosomes. Furthermore, the persistent culture developed sensitivity to metronidazole and resistance against ofloxacin. These results suggest that M. smegmatis might be useful as a fast growing non-pathogenic model for comparative molecular analyses of mycobacterial dormancy.