In vitro susceptibility of human and environmental isolates of Mycobacterium avium, M. intracellulare, and M. scrofulaceum to heavy-metal salts and oxyanions

Abstract

Because of the widespread distribution of Mycobacterium intracellulare and M. scrofulaceum in southeastern U.S. waters, the susceptibility of members of these species to heavy-metal salts and oxyanions was investigated. Isolates with abnormally high tolerance to mercuric chloride or cadmium chloride were identified.The Mycobacterium avium, M. intracellulare, and M. scrofulaceum group includes pathogens whose source for human infection appears to be environmental (18). In fact, the great abundance of these mycobacteria in water (5,6) correlates with the high proportioq of residents in counties of the southeastern states who show evidence of prior infection (3,4).Because of the widespread distribution of M. intracellulare and M. scrofulaceum (M. avium is rarely found) in southeastern U.S. waters (5, 6), including the heavy-metalpolluted Chesapeake and Delaware Bays (8, 15), we investigated the susceptibility of members of these species to heavy-metal salts and oxyanions, to identify highly resistant strains that will serve as subjects for studies .of the genetic and physiological basis for resistance. The rationale of the work was based on the observation of Nelson and Colwell (10) who inferred a "possible causal relationship between ambient mercury levels and numbers of mercury-resistant bacteria" in the Chesapeake Bay. Further, resistance in a variety of bacteria to high concentrations of mercury salts (11,12,14), as well as to other heavy metals such as cadmium (16), is due to plasmid-encoded gene functions. Though plasmid DNA in M. avium and M. intracellulare has been shown to be involved in restriction and modification (2) and in colonial variation (9), none of these characteristics are useful genetic markers, unlike metal resistance. Previous studies of the influence of heavy-metal salts on mycobacteria have used only very high concentrations of salts, and these studies have either focused on the effect of these concentrations on colonial characteristics such as pigmentation (13) waters of the eastern United States (5). The basal medium was Middlebrook and Cohn 7H10 agar medium (BBL Microbiology Systems, Cockeysville, Md.) containing 0.5% (vol/vol) glycerol (M7H10), to which was added HgCl2, NiSO4 * 6H20, Co(NO3)2 * 6H20, CdC12 * 21/2H20, AgNO3, CuSO4 5H20, K2CrO4, Na2SeO3, Na2SeO4, K2TeO3, or K2TeO4 to achieve final metal salt concentrations of 10-6 to 10-3 M. Basal medium was prepared and sterilized by autoclaving, and appropriate amounts of filter-sterilized stock solutions of the metal salts were added before pouring plates, with the pH adjusted to that of M7H10 (pH 6.6) if necessary. For susceptibility tests, the isolates were grown to the mid-log phase (ca. 2 x 108 cells per ml) in Middlebrook and Cohn liquid medium 7H9 (BBL) containing 0.5% (vol/ vol) glycerol and 10% (vol/vol) OADC enrichment (BBL) in screw-capped tubes. A loop containing 0.01 ml of each culture was streaked on the surface of the metal-containing and metal-free (control) 7H10 agar medium. The plates were incubated at 30°C in candl...

Epidemiology of Infection by Nontuberculous Mycobacteria

“…Nontuberculous mycobacteria are relatively resistant to heavy metals and oxyanions (73,149,475). In fact, some isolates of M. avium, M. intracellulare, or M. scrofulaceum were abnormally resistant to cadmium, mercury, silver, and tellurite (149).…”

“…Nontuberculous mycobacteria are relatively resistant to heavy metals and oxyanions (73,149,475). In fact, some isolates of M. avium, M. intracellulare, or M. scrofulaceum were abnormally resistant to cadmium, mercury, silver, and tellurite (149). The fact that five of seven mercury-resistant isolates were isolated from water samples collected in regions of reported heavy metal pollution (149) demonstrates that mycobacterial physiology is a determinant of their ecology and geographic distribution.…”

“…In fact, some isolates of M. avium, M. intracellulare, or M. scrofulaceum were abnormally resistant to cadmium, mercury, silver, and tellurite (149). The fact that five of seven mercury-resistant isolates were isolated from water samples collected in regions of reported heavy metal pollution (149) demonstrates that mycobacterial physiology is a determinant of their ecology and geographic distribution. Furthermore, metal metabolism and metal requirements directly influence M. avium complex populations (e.g., M. avium numbers in natural waters directly correlate with zinc levels [291]).…”

Factors Influencing the Chlorine Susceptibility of Mycobacterium avium , Mycobacterium intracellulare , and Mycobacterium scrofulaceum

“…Not only is M. avium resistant to chlorine, but it and other MAIS organisms are also resistant to heavy metals and oxyanions (12), antibiotics (30), and disinfectants such as quaternary ammonium compounds, phenols, and glutaraldehyde (3,6). Because broad-spectrum antimicrobial resistance is expected to be a major determinant of the epidemiology of opportunistic environmental mycobacteria, I have initiated a long-term study whose objective is to identify the cellular and molecular bases for resistance.…”

Mercuric reductase activity and evidence of broad-spectrum mercury resistance among clinical isolates of rapidly growing mycobacteria

“…Since that time, mercury resistance has come to be recognized as widespread among procaryotic species isolated from human and environmental sources (4,14,23). In some areas (especially Japan), resistance to heavy metals such as mercury is as common as or more common than resistance to antibiotics (10)(11)(12)14).…”