The response of bacterial luminescence to an inhibitor of fatty acid biosynthesis

On the possible ecological roles of antimicrobials

The Introduction of antibiotics into the clinical use in the middle of the 20th century had a profound impact on modern medicine and human wellbeing. The contribution of these wonder molecules to public health and science is hard to overestimate. Much research has informed our understanding of antibiotic mechanisms of action and resistance at inhibitory concentrations in the lab and in the clinic. Antibiotics, however, are not a human invention as most of them are either natural products produced by soil microorganisms or semisynthetic derivatives of natural products. Because we use antibiotics to inhibit the bacterial growth, it is generally assumed that growth inhibition is also their primary ecological function in the environment. Nevertheless, multiple studies point to diverse nonlethal effects that are exhibited at lower levels of antibiotics. Here we review accumulating evidence of antibiosis and of alternative functions of antibiotics exhibited at subinhibitory concentrations. We also speculate on how these effects might alter phenotypes, fitness, and community composition of microbes in the context of the environment and suggest directions for future research.

Luminescence-based systems for detection of bacteria in the environment

The development of techniques for detection and tracking of microorganisms in natural environments has been accelerated by the requirement for assessment of the risks associated with environmental release of genetically engineered microbial inocula. Molecular marker systems are particularly appropriate for such studies and luminescence-based markers have the broadest range of applications, involving the introduction of prokaryotic (lux) or eukaryotic (luc) genes for the enzyme luciferase. Lux or luc genes can be detected on the basis of unique DNA sequences by gene probing and PCR amplification, but the major advantage of luminescence-based systems is the ability to detect light emitted by marked organisms or by luciferase activity in cell-free extracts. Luminescent colonies can be detected by eye, providing distinction from colonies of indigenous organisms, and the sensitivity of plate counting can be increased greatly by CCD imaging. Single cells or microcolonies of luminescent organisms can also be detected in environmental samples by CCD image-enhanced microscopy, facilitating study of their spatial distribution. The metabolic activity of luminescence-marked populations can be quantified by luminometry and does not require extraction of cells or laboratory growth. Metabolic activity, and potential activity, of marked organisms therefore can be measured during colonization of soil particles and plant material in real time without disturbing the colonization process. In comparison with traditional activity techniques, luminometry provides significant increases in sensitivity, accuracy, and, most importantly, selectivity, as activity can be measured in the presence of indigenous microbial communities. The sensitivity, speed, and convenience of luminescence measurements make this a powerful technique that is being applied to the study of an increasingly wide range of ecological problems. These include microbial survival and recovery, microbial predation, plant pathogenicity, phylloplane and rhizosphere colonization and reporting of gene expression in environmental samples.

Inhibition of Vibrio harveyi bioluminescence by cerulenin: in vivo evidence for covalent modification of the reductase enzyme involved in aldehyde synthesis

Bacterial bioluminescence is very sensitive to cerulenin, a fungal antibiotic which is known to inhibit fatty acid synthesis. When Vibrio harveyi cells pretreated with cerulenin were incubated with [3H]myristic acid in vivo, acylation of the 57-kilodalton reductase subunit of the luminescence-specific fatty acid reductase complex was specifically inhibited. In contrast, in vitro acylation of both the synthetase and transferase subunits, as well as the activities of luciferase, transferase, and aldehyde dehydrogenase, were not adversely affected by cerulenin. Light emission of wild-type V. harveyi was 20-fold less sensitive to cerulenin at low concentrations (10 micrograms/ml) than that of the dark mutant strain M17, which requires exogenous myristic acid for luminescence because of a defective transferase subunit. The sensitivity of myristic acid-stimulated luminescence in the mutant strain M17 exceeded that of phospholipid synthesis from [14C]acetate, whereas uptake and incorporation of exogenous [14C]myristic acid into phospholipids was increased by cerulenin. The reductase subunit could be labeled by incubating M17 cells with [3H]tetrahydrocerulenin; this labeling was prevented by preincubation with either unlabeled cerulenin or myristic acid. Labeling of the reductase subunit with [3H]tetrahydrocerulenin was also noted in an aldehyde-stimulated mutant (A16) but not in wild-type cells or in another aldehyde-stimulated mutant (M42) in which [3H]myristoyl turnover at the reductase subunit was found to be defective. These results indicate that (i) cerulenin specifically and covalently inhibits the reductase component of aldehyde synthesis, (ii) this enzyme is partially protected from cerulenin inhibition in the wild-type strain in vivo, and (iii) two dark mutants which exhibit similar luminescence phenotypes (mutants A16 and M42) are blocked at different stages of fatty acid reduction.