Salmonella enterica Serovar Typhimurium Promotes Transmission through Population Divergence
During their life cycle, pathogenic Salmonella strains often need to navigate between two distinct habitats—the host and the environment. MacKenzie et al. (p. 2312–2326) provide evidence that Salmonella enterica serovar Typhimurium has adopted a strategy of population divergence to promote transmission; multicellular aggregates can resist harsh environmental conditions until an opportunity for infection arises, whereas single cells express virulence factors and are immediately able to infect a host. In addition to connecting previously disparate phenotypes, this work lays the foundation for examining differences between host generalist and host-adapted Salmonella strains and their modes of transmission.
Critical Role for Toll-Like Receptor 2, Tpl2, and Extracellular Signal-Regulated Kinase Signaling in Macrophages To Suppress Th1 Responses against Mycobacterium tuberculosis
Mycobacterium tuberculosis induces Th1 responses that are protective but fail to provide sterilizing immunity. Richardson et al. (p. 2242–2254) report that Toll-like receptor 2 (TLR2) is required for M. tuberculosis to activate the Tpl2 kinase and extracellular signal-regulated kinase (ERK) pathway in macrophages. The TLR2-Tpl2-ERK pathway induces interleukin-10, reduces class II major histocompatibility complex antigen presentation, suppresses the Th1-polarizing cytokine interleukin-12, and inhibits the ability of macrophages to stimulate antigen-specific Th1 responses. These findings highlight a novel role for Tpl2 and ERK within macrophages in dampening Th1 cell responses, providing a potential mechanism for immune evasion by M. tuberculosis.
Human Cathelicidin LL-37 Can Be Inactivated by Candida albicans Aspartic Proteases at Sites of Infection
Human cathelicidin LL-37, an efficient antimicrobial peptide, is commonly used by neutrophils or epithelial cells against the opportunistic fungal pathogen Candida albicans at infection sites. This pathogen utilizes several factors, such as adhesins, secreted hydrolases, or biofilm formation, to colonize distinct niches in the host. A family of 10 secreted aspartic proteases (SAPs) belongs to major candidal virulence factors. Rapala-Kozik et al. (p. 2518–2530) show that these enzymes gradually degrade LL-37, eliminating the antifungal and immunomodulatory properties that allow it to influence the life span and chemotaxis of neutrophils. These features of SAPs should be taken into account during development of new peptide-based antifungal drugs.
Getting Closer to Revealing the Secret Intracellular Life of Chlamydia
The identification 15 years ago of CPAF (chlamydial protease-like activity factor) as a Chlamydia-secreted protease captured the imagination of many chlamydia researchers, leading to extensive characterization of this unique serine protease. However, a new study by Yang et al. (p. 2234–2241) has revealed that many of the CPAF critical residues identified in cell-free systems are not necessarily critical for CPAF inside Chlamydia-infected cells, suggesting that the intracellular strategies used by Chlamydia may not be accurately measured in surrogate systems. This work suggests that by investigating how CPAF activates, secretes, and binds substrates in the context of chlamydial infection, we can learn the true faces of the intracellular life of Chlamydia.
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