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Interbacterial antagonism involves all major phyla, occurs across the full range of ecological niches, and has great significance for the environment, clinical arena, and agricultural and industrial sectors. Though the earliest insight into interbacterial antagonism traces back to the discovery of antibiotics, a paradigm shift happened when it was learned that protein secretion systems (e.g., types VI and IV secretion systems) deliver toxic "effectors" against competitors. However, a link between interbacterial antagonism and the Gram-negative type II secretion system (T2SS), which exists in many pathogens and environmental species, is not evident in prior reviews on bacterial competition or T2SS function. A current examination of the literature revealed four examples of a T2SS or one of its known substrates having a bactericidal activity against a Gram-positive target or another Gram-negative. When further studied, the T2SS effectors proved to be peptidases that target the peptidoglycan of the competitor. There are also reports of various bacteriolytic enzymes occurring in the culture supernatants of some other Gram-negative species, and a link between these bactericidal activities and T2SS is suggested. Thus, a T2SS can be a mediator of interbacterial antagonism, and it is possible that many T2SSs have antibacterial outputs. Yet, at present, the T2SS remains relatively understudied for its role in interbacterial competition. Arguably, there is a need to analyze the T2SSs of a broader range of species for their role in interbacterial antagonism. Such investigation offers, among other things, a possible pathway toward developing new antimicrobials for treating disease., Competing Interests: The author declares no conflict of interest.

ABSTRACT Secretion of specific proteins contributes to pathogenesis and immune responses in tuberculosis and other bacterial infections, yet the kinetics of protein secretion and fate of secreted proteins in vivo are poorly understood. We generated new monoclonal antibodies that recognize the Mycobacterium tuberculosis secreted protein Ag85B and used them to establish and characterize a sensitive enzyme-linked immunosorbent assay (ELISA) to quantitate Ag85B in samples generated in vitro and in vivo. We found that nutritional or culture conditions had little impact on the secretion of Ag85B and that there is considerable variation in Ag85B secretion by distinct strains in the M. tuberculosis complex: compared with the commonly used H37Rv strain (lineage 4), Mycobacterium africanum (lineage 6) secretes less Ag85B, and two strains from lineage 2 secrete more Ag85B. We also used the ELISA to determine that the rate of secretion of Ag85B is 10- to 100-fold lower than that of proteins secreted by Gram-negative and Gram-positive bacteria, respectively. ELISA quantitation of Ag85B in lung homogenates of M. tuberculosis H37Rv-infected mice revealed that although Ag85B accumulates in the lungs as the bacterial population expands, the amount of Ag85B per bacterium decreases nearly 10,000-fold at later stages of infection, coincident with the development of T cell responses and arrest of bacterial population growth. These results indicate that bacterial protein secretion in vivo is dynamic and regulated, and quantitation of secreted bacterial proteins can contribute to the understanding of pathogenesis and immunity in tuberculosis and other infections. IMPORTANCE Bacterial protein secretion contributes to host-pathogen interactions, yet the process and consequences of bacterial protein secretion during infection are poorly understood. We developed a sensitive ELISA to quantitate a protein (termed Ag85B) secreted by M. tuberculosis and used it to find that Ag85B secretion occurs with slower kinetics than for proteins secreted by Gram-positive and Gram-negative bacteria and that accumulation of Ag85B in the lungs is markedly regulated as a function of the bacterial population density. Our results demonstrate that quantitation of bacterial proteins during infection can reveal novel insights into host-pathogen interactions.

Secretion of specific proteins contributes to pathogenesis and immune responses in tuberculosis and other bacterial infections, yet the kinetics of protein secretion and fate of secreted proteins in vivo are poorly understood. We generated new monoclonal antibodies that recognize the Mycobacterium tuberculosis secreted protein Ag85B and used them to establish and characterize a sensitive enzyme-linked immunosorbent assay (ELISA) to quantitate Ag85B in samples generated in vitro and in vivo We found that nutritional or culture conditions had little impact on the secretion of Ag85B and that there is considerable variation in Ag85B secretion by distinct strains in the M. tuberculosis complex: compared with the commonly used H37Rv strain (lineage 4), Mycobacterium H37Rv-infected mice revealed that although Ag85B accumulates in the lungs as the bacterial population expands, the amount of Ag85B per bacterium decreases nearly 10,000-fold at later stages of infection, coincident with the development of T cell responses and arrest of bacterial population growth. These results indicate that bacterial protein secretion africanum (lineage 6) secretes less Ag85B, and two strains from lineage 2 secrete more Ag85B. We also used the ELISA to determine that the rate of secretion of Ag85B is 10- to 100-fold lower than that of proteins secreted by Gram-negative and Gram-positive bacteria, respectively. ELISA quantitation of Ag85B in lung homogenates of M. tuberculosis Bacterial protein secretion contributes to host-pathogen interactions, yet the process and consequences of bacterial protein secretion during infection are poorly understood. We developed a sensitive ELISA to quantitate a protein (termed Ag85B) secreted by in vivo is dynamic and regulated, and quantitation of secreted bacterial proteins can contribute to the understanding of pathogenesis and immunity in tuberculosis and other infections. IMPORTANCE Bacterial protein secretion contributes to host-pathogen interactions, yet the process and consequences of bacterial protein secretion during infection are poorly understood. We developed a sensitive ELISA to quantitate a protein (termed Ag85B) secreted by M. tuberculosis and used it to find that Ag85B secretion occurs with slower kinetics than for proteins secreted by Gram-positive and Gram-negative bacteria and that accumulation of Ag85B in the lungs is markedly regulated as a function of the bacterial population density. Our results demonstrate that quantitation of bacterial proteins during infection can reveal novel insights into host-pathogen interactions., (Copyright © 2019 Ernst et al.)