Strains of Mycobacterium tuberculosis differ in affinity for human osteoblasts and alveolar cells in vitro

Abstract

Although the lung is the primary site of infection of tuberculosis, Mycobacterium tuberculosis is capable of causing infection at other sites. In 5-10 % such extra-pulmonary tuberculosis is located in bone tissue of the spine. It is unknown whether host or microbial factors are responsible for the site where extra-pulmonary tuberculosis manifests itself. One MDR isolate belonging to strain F28, one susceptible F11 and one isolate each of susceptible, MDR and XDR F15/LAM4/KZN were cultured in Middlebrook 7H9 media. Human osteoblasts (SaOS-2) and human alveolar epithelial cells (A549) were exposed to these different isolates of M. tuberculosis and invasion capacity and intra-cellular multiplication rates were established. Mouse macrophage (MHS) cells exposed to M. tuberculosis H37Rv served as control. The invasion capacity of F15/LAM4/KZN representatives increased with the level of resistance. The F28 MDR strain showed similar invasion capacity as the XDR F15/LAM4/KZN for pulmonary epthelial cells, whilst the fully susceptible F11 strain displayed a propensity for osteoblasts. The differences observed may in part explain why certain strains are able to cause infection at specific extra-pulmonary sites. We postulated that the development of extra-pulmonary tuberculosis depends on the ability of the microbe to pass effectively through the alveolar epithelial lining and its affinity for cells other than those in pulmonary tissue.

Keywords: Extra-pulmonary tuberculosis; Human alveolar epithelial cells (A549); Human osteoblasts (SaOS-2); Mycobacterium tuberculosis; XDR F15/LAM4/KZN.

Fig. 1

Comparison of the invasion capacity of different strains of Mycobacterium tuberculosis within human osteoblasts (SaOS-2) cells, epithelial cells (A549) and murine alveolar macrophages (MHS) at MOI 9-12. After 2 h of infection cells were incubated with antibiotics to kill extracellular bacteria. Cells were lysed and plated on 7H11 agar to determine intracellular CFU. Data are the mean (±SD) of three separate experiments. Different superscripts in different cell lines are indication of significance (P < 0.05)

Fig. 2

Intracellular growth of different Mycobacterium tuberculosis strains within a human osteoblasts (SaOS-2) cells, b human epithelial cells (A549) and c murine alveolar macrophages (MHS). After infection at MOI 9-12, cells were incubated with antibiotics to kill extracellular bacteria. Cells were further cultured in their specific media on 37 °C in 5 % CO₂ atmosphere for different time hours to see the growth of different strains in different cell types. After specific time points cells were lysed and plated on 7H11 agar to determine intracellular CFU/ml. Data are the mean (±SD) of three separate experiments. (*) Indicates significant value (P < 0.05)