Engineered bacteria for therapeutic applications would benefit from control mechanisms that confine the growth of the bacteria within specific tissues or regions in the body. Here we show that the tropism of engineered bacteria can be enhanced by coupling bacterial growth with genetic circuits that sense oxygen, pH or lactate through the control of the expression of essential genes. Bacteria that were engineered with pH or oxygen sensors showed preferential growth in physiologically relevant acidic or oxygen conditions, and reduced growth outside the permissive environments when orally delivered to mice. In syngeneic mice bearing subcutaneous tumours, bacteria engineered with both hypoxia and lactate biosensors coupled through an AND gate showed increased tumour specificity. The multiplexing of genetic circuits may be more broadly applicable for enhancing the localization of bacteria to specified niches. The tropism of engineered bacteria in vivo can be enhanced by genetic circuits, linked by AND-gate logic, that couple bacterial sensing and growth.

Enhancing the tropism of bacteria via genetically programmed biosensors.

Acidity is one of the principal physicochemical factors that influence the behavior of microorganisms in any environment, and their response to it often determines their ability to grow and survive. Preventing the growth and survival of pathogenic bacteria or, conversely, promoting the growth of bacteria that are useful (in biotechnology and food production, for example), might be improved considerably by a deeper understanding of the protective responses that these microorganisms deploy in the face of acid stress. In this review, we survey the molecular mechanisms used by two unrelated bacterial species in their response to low pH stress. We chose to focus on two well-studied bacteria, Escherichia coli (phylum Proteobacteria) and Listeria monocytogenes (phylum Firmicutes), that have both evolved to be able to survive in the mammalian gastrointestinal tract. We review the mechanisms that these species use to maintain a functional intracellular pH as well as the protective mechanisms that they deploy to prevent acid damage to macromolecules in the cells. We discuss the mechanisms used to sense acid in the environment and the regulatory processes that are activated when acid is encountered. We also highlight the specific challenges presented by organic acids. Common themes emerge from this comparison as well as unique strategies that each species uses to cope with acid stress. We highlight some of the important research questions that still need to be addressed in this fascinating field.

https://www.mdpi.com/2073-4425/11/11/1330/pdf

Comparative Review of the Responses of Listeria monocytogenes and Escherichia coli to Low pH Stress.

Background Acid-tolerant enteric pathogens can evade small intestinal acid barriers, colonize and infect the intestinal tract. However, broad-spectrum antibiotics are not the best therapeutic strategy because of the disruption of intestinal flora caused by its indiscriminate antimicrobial activity against beneficial and harmful bacteria. So that is what inspired us to combine pH regulation with nanotechnology to develop a pH-triggered site-targeted antimicrobial peptide with entrapping function. Results A pH-triggered dual biological functional self-assembled peptide (SAP) was designed according to the features of amino-acid building blocks and the diagonal cation-π interaction principle. The results of characterization experiments showed that changes in pH conditions could trigger microstructural transformation of the nanopeptide from nanospheres to nanofibers. The subsequent antibacterial and toxicity experiments determined that SAP had great antimicrobial activity against Escherichia coli, Salmonella typhimurium, Listeria monocytogenes, and Bacillus cereus above 15.6 μg/mL under acidic conditions by disrupting bacterial membrane integrity, excellent biocompatibility in vitro even at 250 μg/mL and high tolerance in physical environment. Moreover, at peptide concentrations greater than 62.5 μg/mL, SAP showed the entrapment property, which played an important role in phagocytic clearance in infection forces. Meanwhile, the in vivo results revealed that SAP possessed excellent therapeutic effect and good biosafety. Conclusions Our study revealed the antibacterial activity of a short β-hairpin forming self-assembled peptide, and established an innovative design strategy for peptide-based nanomaterials and a new treatment strategy for gastrointestinal bacterial infections.

/pdf/de-novo-design-of-a-ph-triggered-self-assembled-b-hairpin-132wpatd64.pdf

De novo design of a pH-triggered self-assembled β-hairpin nanopeptide with the dual biological functions for antibacterial and entrapment.

As natural metabolites, organic acids have been widely applied in food, pharmaceutical, and bio-based materials industries. Particularly, the short-chain organic acids, including C2, C3, C4, C5, and C6 organic acids, are necessary intermediate metabolites in cells and are also alternatives to some commercial chemical products. As the necessary metabolites in cells, most major short-chain organic acids can be produced through microbial fermentation. Specifically, with the development of synthetic biology, metabolic engineering could endow cells with the ability to produce more short-chain organic acid products including propionic acid, pyruvate, lactic acid, 3-hydroxypropionic acid, malic acid, succinic acid, fumaric acid, butyric acid, itaconic acid, α-ketoglutaric acid, glutaric acid, citric acid, gluconic acid, muconic acid, adipic acid, xylonic acid, and so on. The recent advances in the biological production of short-chain organic acids, as well as the challenges and perspectives, are summarized in this review to promote the generation of microbial cell factories for the production of short-chain organic acids.Key points• Outlines the production strategy of short-chain organic acids• Provide guidance for efficient synthesis of short-chain organic acids• Impacts the necessary factor of acid resistance on the successful production of host cells.

Current advance in biological production of short-chain organic acid

Adaptive evolution improves acid tolerance and succinic acid production in Actinobacillus succinogenes

The ability to grow at moderate acidic conditions (pH 4.0–5.0) is important to Escherichia coli colonization of the host’s intestine. Several regulatory systems are known to control acid resistance in E. coli, enabling the bacteria to survive under acidic conditions without growth. Here, we characterize an acid-tolerance response (ATR) system and its regulatory circuit, required for E. coli exponential growth at pH 4.2. A two-component system CpxRA directly senses acidification through protonation of CpxA periplasmic histidine residues, and upregulates the fabA and fabB genes, leading to increased production of unsaturated fatty acids. Changes in lipid composition decrease membrane fluidity, F0F1-ATPase activity, and improve intracellular pH homeostasis. The ATR system is important for E. coli survival in the mouse intestine and for production of higher level of 3-hydroxypropionate during fermentation. Furthermore, this ATR system appears to be conserved in other Gram-negative bacteria. The ability to grow at acidic pH is crucial for E. coli colonization of the host’s intestine. Here, the authors identify an acid-tolerance response system that is important for E. coli exponential growth at pH 4.2, survival in the mouse intestine, and production of 3-hydroxypropionate during fermentation.

/pdf/an-acid-tolerance-response-system-protecting-exponentially-4o59gmkla4.pdf

Qingsheng Qi

Papers

An acid-tolerance response system protecting exponentially growing Escherichia coli.