Seattle Biomedical Research Institute, 4 Nickerson Street, WA 98109, Seattle, USA1
Kuzell Institute of Arthritis and Infectious Disease, San Francisco, CA, USA2
Author for correspondence: Gerard A. Cangelosi. Tel: +1 206 284 8846. Fax: +1 206 284 0313. e-mail: gcang{at}sbri.org
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ABSTRACT |
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Keywords: AIDS, drug resistance, virulence, motility, phenotypic switching
Abbreviations: CR, Congo red; DR, dark red; relative Congo red binding; RO, red opaque; RT, red transparent; WO, white opaque; WT, white transparent; ZOI, zone of inhibition
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INTRODUCTION |
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M. avium is intrinsically resistant to many drugs used to treat other mycobacterial infections. This resistance is usually ascribed to the impermeability of its lipid-rich cell wall, but it may also occur through other mechanisms (Inderlied et al., 1993 ; Jarlier & Nikaido, 1994
; Mdluli et al., 1998
; Peloquin, 1997
; Portillo-Gomez et al., 1995
). Effective drugs such as clarithromycin, rifabutin, fluoroquinolones and amikacin must be administered in combination to prevent the possible emergence of drug resistance. For unknown reasons, transparent variants are more drug-resistant than opaque variants. Therefore, laboratory drug-susceptibility testing must be carried out on isolated transparent colonies. However, even with this precaution there is considerable assay-to-assay variability, suggesting that additional morphotypic switches may also affect drug susceptibility (Heifets, 1996
; Inderlied & Nash, 1996
; Sison et al., 1996
).
We recently described a new phenotypic variation detectable among opaque colonies of M. avium grown on agar media containing the lipoprotein stain Congo red (CR). About one-third of clinical isolates examined formed red opaque (RO) as well white opaque (WO) colonies under these conditions, while the remaining two-thirds formed only red and, in some cases, pink opaque colonies (Cangelosi et al., 1999 ). RFLP analysis using an IS1245 probe showed that the RO and WO variants were segregants of single strains, not distinct strains. WO clones were found to be more resistant to antimycobacterial drugs than were their RO counterparts. The physiological basis for redwhite variation has not yet been determined.
In contrast to opaque colonies, which were vividly heterogeneous on CR plates, transparent colonies of clinical isolates had uniform pale pink colours that did not differ much from the underlying CR agar. For this reason, our previous study did not reveal whether redwhite segregation also exists within the more clinically significant transparent morphotype. The present report shows that it does. Transparent derivatives of WO clones (WT) bound less CR, were more resistant to multiple antibiotics and were better able to survive within human macrophages than were transparent derivatives of RO clones (RT). We also examined the stability of the white and red morphotypes and the effects of redwhite switching on growth and spreading in vitro.
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METHODS |
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Transparent phase variants of strain HMC10 were isolated without antibiotic selection, as follows. RO and WO cells grown on MAG plates were suspended in autoclaved deionized water to a density of 107 c.f.u. ml-1. The suspensions were incubated without agitation at room temperature for 4 d and then spread onto MAG-CR plates to obtain isolated colonies. Following this procedure, transparent colonies accounted for 130% of total colonies, compared to <1% prior to incubation in water (data not shown). Three transparent clones derived from the RO parent were subcultured onto MAG-CR plates and examined by stereoscopic microscopy for light transmission to confirm their stable conversion to the transparent phase. These clones were designated HMC10-RT1, -RT2 and -RT3. Three transparent clones derived from the WO parent, HMC10-WT1, -WT2 and -WT3, were isolated by the same process.
In vitro drug susceptibility tests.
For comparing the drug susceptibilities of isolated colony-type variants, we have found that agar diffusion assays give more consistent results than broth-based susceptibility tests because colony morphotypes are clearly visible on agar plates, and spurious results arising from morphotypic switching are thereby avoided. Accordingly, clarithromycin and rifampin susceptibilities were assessed by using disc-diffusion assays. Bacteria were spread onto MAG-CR plates, then filter paper discs impregnated with 15 µg clarithromycin (Remel Microbiology Products) or 25 µg rifampin (BBL, Difco-Becton Dickinson) were placed onto the plates. Disc-diffusion plates were incubated in polyethylene bags for 14 d at 37 °C under 5% CO2 before ZOIs of growth around the discs were measured.
Isolation of human macrophages.
Monocytes were isolated by Histopagne-1077 (Sigma) gradient centrifugation from the white cell concentrate (Blood Center of the Pacific, San Francisco, CA). Mononuclear phagocytes were obtained as previously described (Bermudez et al., 1997 ; Hsu et al., 1995
). Briefly, non-adherent cells were removed by washing after 2 h incubation in RPMI-1640 medium (Gibco-BRL, Life Technologies) supplemented with 10% fetal bovine serum and 2 mM L-glutamine (Sigma). Adherent mononuclear phagocytes differentiate into macrophages after 4 d in culture.
Infection of human macrophages.
Macrophage monolayers (5x105 cells) were incubated for 1 h with RO, WO, RT or WT clones of strains HMC02 and HMC10, at a ratio of 10 bacteria per macrophage. Extracellular bacteria were removed from the monolayers by two gentle washes with HBSS warmed to 37 °C. Infected macrophages were incubated in fresh RPMI-1640 medium at 37 °C under 5% CO2. To measure resistance to killing and inhibition of growth by macrophages, the number of intracellular bacteria 4 d after infection was compared to the number of intracellular bacteria 2 h after infection. Intracellular bacteria were harvested after lysis of the macrophages as previously described (Bermudez et al., 1997 ; Hsu et al., 1995
). Lysates were serially diluted and plated onto 7H11 agar plates and incubated at 37 °C. c.f.u. were counted after 10 d.
Sliding motility.
RO, WO, RT and WT clones were picked from frozen stocks or from actively growing (10-d-old) colonies on MAG-CR plates. Using a 1 µl loop, bacteria were inoculated into the centre of 100x15 mm plates containing MAG or MAG-CR semi-solidified with 0·3% agar (Martinez et al., 1999 ). Plates were incubated in sealed polyethylene bags at 37 °C under 5% CO2. Spreading was measured at various time points over the course of 24 weeks.
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RESULTS |
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RT and WT variants differ with regard to drug susceptibility
Drug susceptibility of transparent clones was measured by agar diffusion. As in our previous study (Cangelosi et al., 1999 ), this approach gave more reproducible results than broth-based susceptibility tests because colony morphotypes were clearly visible on plates. In both HMC02 and HMC10, WT clones were significantly more resistant than their RT counterparts to clarithromycin and rifampin (Fig. 2
). Therefore, WT clones of both strains retained the multidrug-resistance properties of their WO progenitors.
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The two morphotypes differed with regard to sliding motility on soft agar. Martinez et al. (1999 ) recently published the first detailed report of sliding motility in Mycobacterium species, including M. avium. Sliding motility was defined as the ability of cell masses to spread over moist surfaces as cell numbers increase. To determine whether redwhite variation affects sliding motility, we inoculated 0·3% agar plates with RO, WO, RT and WT clones of strains HMC02 and HMC10 as described by Martinez et al. (1999
). All variants sometimes developed clear haloes that contained few or no bacterial cells detectable by cultivation. These haloes may have been composed of extracellular material that diffused through the agar. Within the haloes, colonies of red and white variants exhibited different patterns of spreading. As shown in Fig. 4a
, red variants initially spread more aggressively than their white counterparts. For the first 23 weeks of each experiment, WO and WT clones appeared as confined, non-motile colonies with branches that may have spread through small cracks in the agar at the inoculation site. By contrast, RO and RT clones exhibited the distinctive spreading patterns described by Martinez et al. (1999
). RO colonies spread more quickly than did RT colonies. Similar results were observed whether inocula were frozen cell stocks or actively growing cells picked from MAG-CR plates, suggesting that results were not influenced by varying physiological states of the inocula. The presence or absence of CR in the soft agar also did not affect results.
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Directionality of redwhite switching
In our previous study (Cangelosi et al., 1999 ), we plated stocks of 15 clinical isolates of M. avium onto CR agar and reported their staining phenotypes. Prior to plating onto CR agar, these stocks had been passed three to four times on Middlebrook 7H11 agar and LowensteinJensen slants, and some of them had also been cultured once on BACTEC broth. Of the 15 isolates, five formed mixed (RO and WO) colonies, while the remaining ten formed red and/or pink colonies, but no white colonies. Since that study, we have examined some isolates in greater detail to determine the stability of the red and white phenotypes in vitro. Isolated RO clones of mixed strains HMC02 and HMC10 were very stable, forming red, pink, or DR colonies but no white colonies over the course of repeated transfers. Pink and DR variants were not characterized further. RO clones isolated from a mixed laboratory strain, 102, also formed no white colonies. Attempts to use antibiotic selection to isolate white derivatives of HMCO2-RO were unsuccessful. Strains that formed only RO and RT colonies, such as the clinical isolate HMC04 and laboratory strains 104 and 2151, were similarly stable, forming no white colonies after repeated passage on broth and agar media. All of these strains switched back and forth between the opaque and transparent morphotypes at typical frequencies (Woodley & David, 1976
).
In contrast to red variants, plates spread thickly with isolated WO clones frequently formed RO colonies as well as pink and DR colonies. The latter are visible as isolated deep red sectors in Fig. 4(b). RO derivatives of HMC02-WO were isolated and found to be as stable as any other RO clone. When ciprofloxacin susceptibility was measured by using E-tests as described previously (Cangelosi et al., 1999
), these RO derivatives were approximately 10-fold more susceptible than their WO parent, similar to other RO variants of HMC02 (data not shown).
The relative stability of the red morphotype was especially evident when RT and WT variants of HMC02 and HMC10 were allowed to switch back to the opaque morphotype. Opaque derivatives of RT clones were never white, whereas those of WT clones were white, red, pink and DR. These observations indicate that the red morphotype is more stable than the white morphotype in vitro.
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DISCUSSION |
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WT variants were significantly more resistant in vitro to two structurally unrelated drugs, clarithromycin and rifampin, than were their RT counterparts. In the agar diffusion assays we used, the difference between the red and white morphotypes matched or exceeded the difference between the opaque and transparent morphotypes. WT clones also resisted killing by human macrophages, which suggested that they are virulent and capable of contributing to treatment failure by virtue of their drug resistance. The relative sensitivity of RT clones to killing by macrophages does not necessarily indicate that the red morphotype is avirulent. Two-thirds of the M. avium clinical isolates we examined previously did not form white colonies (Cangelosi et al., 1999 ), and strains 104 and 2151, which are virulent in animal models, formed only red colonies when plated on CR agar. Therefore, red variants probably retain at least some virulence properties. Nevertheless, the enhanced survival of WT variants within macrophages may reflect a higher level of pathogenicity. This possibility warrants further analysis using animal models of infection.
Sliding motility is a recently described property of mycobacteria, including M. avium (Martinez et al., 1999 ). The ability to spread over surfaces probably confers a selective advantage on environmental bacteria like M. avium, especially when nutrients become limiting locally. It was in this characteristic that the red morphotype appeared to have an advantage over the white morphotype. This was especially apparent upon extended incubation of white colonies on soft agar plates containing CR. Such colonies appeared white at the centre, surrounded by a spreading halo of red cells, suggesting that a switch to the red morphotype allowed cells to escape from the inoculation site.
White-to-red switching was observed not only on soft agar, but also on standard (1·5% agar) CR plates inoculated with WO or WT cells. By contrast, we have yet to observe red-to-white switching in vitro. It was interesting that despite this strong disequilibrium in favour of the white-to-red switch in vitro, white variants were found in multiple clinical isolates (Cangelosi et al., 1999 ). The white morphotype may be favoured at some stage of infection, disease or treatment. Alternatively, red-to-white switching may be more common in vivo than in vitro. Examination of large numbers of clinical isolates before passage on laboratory media will help to refine these models, especially if the occurrence of white variants can be tested for association with higher rates of treatment failure.
WT clones switched to a variety of colony types, including WO, RO, DR, pink opaque and rough colonies. These colony types were diverse in size as well as in CR staining. Colonies formed by RT clones appeared much more uniform. This may explain why stable red clones such as 2151 and 104 have become widely studied laboratory strains. Strain 104 was the source of DNA for genomic sequencing of M. avium, which is nearing completion by The Institute for Genomic Research (http://www.tigr.org/tdb/mdb/mdb.html). Strain 2151 has been the subject of extensive analysis of cell wall biochemistry (Belisle et al., 1993 ) and other characteristics including motility (Martinez et al., 1999
found strain 2151 to be motile like our red clones). As already noted, transparent variants of these strains are virulent and intrinsically drug resistant. However, they appear to be fixed in a form (red) that is less virulent and less drug resistant than the white form. Analysis of white variants may help us to better understand the full range of M. avium phenotypes that affect pathogenicity, drug resistance and survival in the environment.
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ACKNOWLEDGEMENTS |
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REFERENCES |
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Received 20 June 2000;
revised 11 September 2000;
accepted 14 November 2000.