Evaluation of multidrug-resistant Mycobacterium tuberculosis strains fitness in two in vitro competition models

N YOKOBORI; B LÓPEZ ; G LAFUENTE2; V RITACCO; M SASIAIN

Congreso; Congreso latinoamericano de Microbiología y Sociedad latinoamericana de micobacterias; 2016

In Argentina, multidrug-resistant tuberculosis emerged as a public health problem after the large HIV-related outbreak caused by the M strain in the early ´90s. The M strain is a highly successful Mycobacterium tuberculosis strain of the H2 SIT2 genotype that circulates as pre-extremely drug resistant since the 70?s1 and still persists in our days. The biological determinants of its epidemiological success are unknown. Our hypothesis is that the M strain has an outstanding fitness, despite the numerous drug-resistance conferring mutations acquired. The aim of the current work was to evaluate the fitness of the M strain through in vitro competition assays against a closely related, non-prosperous, strain and the laboratory strain H37Rv. Two clinical isolates with the prototype M strain genotype (6548 from 1998 and 15526 from 2007), the sporadic 410 strain genetically closely related to the M strain (1995; caused a single case; susceptible to kanamycin) and the reference strain H37Rv (pansusceptible) were selected. Two models were assessed: (i) axenic growth competition2 in BACTEC-MGIT 960TM culture system and (ii) mixed infection in human monocyte derived macrophages (MDM) at a MOI of 13. We have previously reported that isolate 6548 was able to overgrow 410 in axenic culture4. In the current work we sought to determine if the same was true for the modern isolate 15526 and in the intracellular milieu. Thus, the following combinations were evaluated in both axenic and intracellular conditions: 6548 vs. 410, 15526 vs. 410, 6548 vs. H37Rv, 15526 vs. H37Rv, 410 vs. H37Rv. At 21 days of culture or 2 and 4 days post-infection, bacterial suspensions were serially diluted and triplicates were plated onto 7H11-AD agar plates with and without antibiotics to determine colony forming units (CFU)/ml for each strain. CFUs were counted after 3 to 5 weeks of incubation. In axenic competition, both isolates from the M outbreak overgrew 410 strain (6548 vs. 410: p