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Bacteriostatic
Activity Of Morone saxatilis Head
Kidney Cells Against Environmental Isolates Of Mycobacterium Sp. Karen G. Burnett1, Martha W. Rhodes2, Louis E. Burnett3
and Howard I. Kator2 1Dept. of
Microbiology and Immunology, Medical University of South Carolina, Charleston,
SC, USA; 2Dept. of Environmental Sciences, Virginia Institute of
Marine Science, Gloucester Point, VA, USA; 3Grice Marine Laboratory,
University of Charleston, Charleston, SC, USA. Although some species of mycobacteria
are well-recognized as opportunistic pathogens in aquacultured fish and other
vertebrates, these slow-growing acid-fast bacteria are seldom linked to overt
disease in feral fish populations.
Within the last three years, granulomatous lesions of the skin and
spleen of striped bass, Morone saxatilis,
taken from the Chesapeake Bay, have routinely stained positive for acid-fast
bacteria. The species most frequently
isolated from these lesions by aseptic tissue sampling include isolates
resembling M. scrofulaceum and nonchromogenic
slow-growing isolates with temperature optima of ca. 23°C. It is unclear at present which if any of
these mycobacterial isolate(s) may underlie the pathology observed in striped
bass. This study compared the
efficiency with which M. saxatilis
head kidney cells could kill or suppress the growth of seven mycobacteria
species previously isolated from symptomatic fish. Adherent head kidney cells (AHK) from asymptomatic striped bass
were challenged with test mycobacteria at two cell:bacteria ratios (1:1 or
1:10) for 4 to 14 days at 25°C.
Surviving bacteria were quantified using a colorimetric assay with the
tetrazolium dye, MTT, at OD590nm.
Anti-bacterial activity was reported as % reduction of OD590nm
in wells with live AHK compared to wells containing bacteria with killed
AHK. Challenge bacterial doses were
confirmed by enumerating colony forming units on Middlebrook 7H10. AHK cells cultured for seven days at the
low challenge ratio (1 AHK: 0.1–1.0 mycobacterium) suppressed outgrowth of M. szulgae, M. gordonae, M. scrofulaceum,
M. interjectum and a fifth
unidentified Mycobacterium sp. to a
similar extent (mean = 80.5%; range 43 to 96%). The same AHK challenge ratio showed significantly lower
bacteriostatic activity against M.
peregrinum and M. marinum [mean
1.5%; range (-38) to 21 %]. At the
high challenge ratio (1 AHK:1-10
mycobacteria), AHK were generally less able to suppress outgrowth of all
mycobacterial isolates [mean 38%; range (-7%) to 91%). These data focus particular attention on M. peregrinum and M. marinum as those isolates best able to resist
bactericidal/bacteriostatic activity of striped bass head kidney macrophages.
(Supported by EPA Agreement R-826399 to KGB).
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