Biofilm Today, Gone Tomorrow: Most bacteria can form complex, matrix-containing multicellular communities known as biofilms, which protect residents from environmental stresses such as antibiotic exposure. However, as biofilms age, nutrients become limiting and waste products accumulate, and biofim disassembly is triggered. Now Kolodkin-Gal et al. (p. 627 ) have found that d -amino acids found in conditioned medium from mature biofilms of Bacillus subtilis prevent biofilm formation and trigger existing biofilm disassembly.

Traditionally, programmed cell death (PCD) is associated with eukaryotic multicellular organisms. However, recently, PCD systems have also been observed in bacteria. Here we review recent research on two kinds of genetic programs that promote bacterial cell death. The first is mediated by mazEF, a toxin–antitoxin module found in the chromosomes of many kinds of bacteria, and mainly studied in Escherichia coli. The second program is found in Bacillus subtilis, in which the skf and sdp operons mediate the death of a subpopulation of sporulating bacterial cells. We relate these two bacterial PCD systems to the ways in which bacterial populations resemble multicellular organisms.

Biofilms are communities of cells held together by a self-produced extracellular matrix typically consisting of protein, exopolysaccharide, and often DNA. A natural signal for biofilm disassembly in Bacillus subtilis is certain d -amino acids, which are incorporated into the peptidoglycan and trigger the release of the protein component of the matrix. d -Amino acids also prevent biofilm formation by the related Gram-positive bacterium Staphylococcus aureus . Here we employed fluorescence microscopy and confocal laser scanning microscopy to investigate how d -amino acids prevent biofilm formation by S. aureus . We report that biofilm formation takes place in two stages, initial attachment to surfaces, resulting in small foci, and the subsequent growth of the foci into large aggregates. d -Amino acids did not prevent the initial surface attachment of cells but blocked the subsequent growth of the foci into larger assemblies of cells. Using protein- and polysaccharide-specific stains, we have shown that d -amino acids inhibited the accumulation of the protein component of the matrix but had little effect on exopolysaccharide production and localization within the biofilm. We conclude that d -amino acids act in an analogous manner to prevent biofilm development in B. subtilis and S. aureus . Finally, to investigate the potential utility of d -amino acids in preventing device-related infections, we have shown that surfaces impregnated with d -amino acids were effective in preventing biofilm growth.

mazEF is a toxin-antitoxin module located on many bacterial chromosomes, including those of pathogens. Here, we report that Escherichia coli mazEF -mediated cell death is a population phenomenon requiring a quorum-sensing molecule that we call the extracellular death factor (EDF). Structural analysis revealed that EDF is a linear pentapeptide, Asn-Asn-Trp-Asn-Asn. Each of the five amino acids of EDF is important for its activity.

Bacillussubtilis forms organized multicellular communities known as biofilms wherein the individual cells are held together by a self-produced extracellular matrix. The environmental signals that promote matrix synthesis remain largely unknown. We discovered that one such signal is impaired respiration. Specifically, high oxygen levels suppressed synthesis of the extracellular matrix. In contrast, low oxygen levels, in the absence of an alternative electron acceptor, led to increased matrix production. The response to impaired respiration was blocked in a mutant lacking cytochromes caa₃ and bc and markedly reduced in a mutant lacking kinase KinB. Mass spectrometry of proteins associated with KinB showed that the kinase was in a complex with multiple components of the aerobic respiratory chain. We propose that KinB is activated via a redox switch involving interaction of its second transmembrane segment with one or more cytochromes under conditions of reduced electron transport. In addition, a second kinase (KinA) contributes to the response to impaired respiration. Evidence suggests that KinA is activated by a decrease in the nicotinamide adenine dinucleotide (NAD⁺)/NADH ratio via binding of NAD⁺ to the kinase in a PAS domain A-dependent manner. Thus, B. subtilis switches from a unicellular to a multicellular state by two pathways that independently respond to conditions of impaired respiration.