Bacterial type VII secretion: An important player in host-microbe and microbe-microbe interactions.

Type VII secretion systems (T7SSs) are poorly understood protein export apparatuses found in mycobacteria and many species of Gram-positive bacteria. To date, this pathway has predominantly been studied in Mycobacterium tuberculosis, where it has been shown to play an essential role in virulence; however, much less studied is an evolutionarily divergent subfamily of T7SSs referred to as the T7SSb. The T7SSb is found in the major Gram-positive phylum Firmicutes where it was recently shown to target both eukaryotic and prokaryotic cells, suggesting a dual role for this pathway in host-microbe and microbe-microbe interactions. In this review, we compare the current understanding of the molecular architectures and substrate repertoires of the well-studied mycobacterial T7SSa systems to that of recently characterized T7SSb pathways and highlight how these differences may explain the observed biological functions of this understudied protein export machine.

Overcoming Acquired and Native Macrolide Resistance with Bicarbonate.

The growing challenge of microbial resistance emphasizes the importance of new antibiotics or reviving strategies for the use of old ones. Macrolide antibiotics are potent bacterial protein synthesis inhibitors with a formidable capacity to treat life-threatening bacterial infections; however, acquired and intrinsic resistance limits their clinical application. In the work presented here, we reveal that bicarbonate is a potent enhancer of the activity of macrolide antibiotics that overcomes both acquired and intrinsic resistance mechanisms. With a focus on azithromycin, a highly prescribed macrolide antibiotic, and using clinically relevant pathogens, we show that physiological concentrations of bicarbonate overcome drug resistance by increasing the intracellular concentration of azithromycin. We demonstrate the potential of bicarbonate as a formulation additive for topical use of azithromycin in treating a murine wound infection caused by Pseudomonas aeruginosa. Further, using a systemic murine model of methicillin-resistant Staphylococcus aureus (MRSA) infection, we demonstrate the potential role of physiological bicarbonate, naturally abundant in the host, to enhance the activity of azithromycin against macrolide-resistant MRSA. In all, our findings suggest that macrolide resistance, observed in the clinical microbiology laboratory using standard culturing techniques, is a poor predictor of efficacy in the clinic and that observed resistance should not necessarily hamper the use of macrolides. Whether as a formulation additive for topical use or as a natural component of host tissues, bicarbonate is a powerful potentiator of macrolides with the capacity to overcome drug resistance in life-threatening bacterial infections.

An interbacterial toxin inhibits target cell growth by synthesizing (p)ppApp.

Bacteria have evolved sophisticated mechanisms to inhibit the growth of competitors1. One such mechanism involves type VI secretion systems, which bacteria can use to inject antibacterial toxins directly into neighbouring cells. Many of these toxins target the integrity of the cell envelope, but the full range of growth inhibitory mechanisms remains unknown2. Here we identify a type VI secretion effector, Tas1, in the opportunistic pathogen Pseudomonas aeruginosa. The crystal structure of Tas1 shows that it is similar to enzymes that synthesize (p)ppGpp, a broadly conserved signalling molecule in bacteria that modulates cell growth rate, particularly in response to nutritional stress3. However, Tas1 does not synthesize (p)ppGpp; instead, it pyrophosphorylates adenosine nucleotides to produce (p)ppApp at rates of nearly 180,000 molecules per minute. Consequently, the delivery of Tas1 into competitor cells drives rapid accumulation of (p)ppApp, depletion of ATP, and widespread dysregulation of essential metabolic pathways, thereby resulting in target cell death. Our findings reveal a previously undescribed mechanism for interbacterial antagonism and demonstrate a physiological role for the metabolite (p)ppApp in bacteria.