Using deuterium as an isotopic tracer to study the energy metabolism of infective juveniles of Steinernema carpocapsae under aerobic conditions

Ready or Not: Microbial Adaptive Responses in Dynamic Symbiosis Environments

In mutually beneficial and pathogenic symbiotic associations, microbes must adapt to the host environment for optimal fitness. Both within an individual host and during transmission between hosts, microbes are exposed to temporal and spatial variation in environmental conditions. The phenomenon of phenotypic variation, in which different subpopulations of cells express distinctive and potentially adaptive characteristics, can contribute to microbial adaptation to a lifestyle that includes rapidly changing environments. The environments experienced by a symbiotic microbe during its life history can be erratic or predictable, and each can impact the evolution of adaptive responses. In particular, the predictability of a rhythmic or cyclical series of environments may promote the evolution of signal transduction cascades that allow preadaptive responses to environments that are likely to be encountered in the future, a phenomenon known as adaptive prediction. In this review, we summarize environmental variations known to occur in some well-studied models of symbiosis and how these may contribute to the evolution of microbial population heterogeneity and anticipatory behavior. We provide details about the symbiosis between Xenorhabdus bacteria and Steinernema nematodes as a model to investigate the concept of environmental adaptation and adaptive prediction in a microbial symbiosis.

Examination of Xenorhabdus nematophila lipases in pathogenic and mutualistic host interactions reveals a role for xlpA in nematode progeny production

Xenorhabdus nematophila is a gammaproteobacterium and broad-host-range insect pathogen. It is also a symbiont of Steinernema carpocapsae, the nematode vector that transports the bacterium between insect hosts. X. nematophila produces several secreted enzymes, including hemolysins, lipases, and proteases, which are thought to contribute to virulence or nutrient acquisition for the bacterium and its nematode host in vivo. X. nematophila has two lipase activities with distinct in vitro specificities for Tween and lecithin. The gene encoding the Tween-specific lipase, xlpA, has been identified and is not required for X. nematophila virulence in one insect host, the tobacco hornworm Manduca sexta. However, the gene encoding the lecithin-specific lipase activity is not currently known. Here, we identify X. nematophila estA, a gene encoding a putative lecithinase, and show that an estA mutant lacks in vitro lipase activity against lecithin but has wild-type virulence in Manduca sexta. X. nematophila secondary-form phenotypic variants have higher in vitro lecithinase activity and estA transcript levels than do primary-form variants, and estA transcription is negatively regulated by NilR, a repressor of nematode colonization factors. We establish a role for xlpA, but not estA, in supporting production of nematode progeny during growth in Galleria mellonella insects. Future research is aimed at characterizing the biological roles of estA and xlpA in other insect hosts.

Masters of conquest and pillage: Xenorhabdus nematophila global regulators control transitions from virulence to nutrient acquisition

Invertebrate animal models are experimentally tractable and have immunity and disease symptoms that mirror those of vertebrates. Therefore they are of particular utility in understanding fundamental aspects of pathogenesis. Indeed, artificial models using human pathogens and invertebrate hosts have revealed conserved and novel molecular mechanisms of bacterial infection and host immune responses. Additional insights may be gained from investigating interactions between invertebrates and pathogens they encounter in their natural environments. For example, enteric bacteria in the genera Photorhabdus and Xenorhabdus are pathogens of insects that also mutualistically associate with nematodes in the genera Heterorhabditis and Steinernema respectively. These bacteria serve as models to understand naturally occurring symbiotic associations that result in disease in or benefit for animals. Xenorhabdus nematophila is the best-studied species of its genus with regard to the molecular mechanisms of its symbiotic associations. In this review, we summarize recent advances in understanding X. nematophila-host interactions. We emphasize regulatory cascades involved in coordinating transitions between various stages of the X. nematophila life cycle: infection, reproduction and transmission.

Purification and characterization of α-amylase from the infective juveniles of the nematode Heterorhabditis bacteriophora

Glycogen content and alpha-amylase activity were estimated in the infective juveniles (IJs) of Heterorhabditis bacteriophora at different times of storage. The glycogen content declined from 5.8 to 2.5 ng/IJ during storage for 40 days at 27 degrees C. The change in glycogen content coincided with the change of alpha-amylase activity during storage. alpha-Amylase was purified from IJs at zero time of storage by ion exchange chromatography and gel filtration. Ion exchange chromatography resolved alpha-amylase into three isoenzymes. The major isoenzyme alpha-amylase I had the highest specific activity and was purified to homogeneity. A molecular mass of 46-47 kDa was estimated for both the native and denatured enzyme, suggesting that the enzyme is monomeric. The Km values were 6.5 and 9.6 mg/ml using starch and glycogen as substrates, respectively. alpha-Amylase I showed optimum activity at pH 7.0 and had an optimum temperature of 40 degrees C. The enzyme was unstable at temperatures above 40 degrees C. The enzyme activity was severely inhibited by EDTA, p-CMB and iodoacetic acid, but potentiated by CaCl2 and NaCl. These results are discussed and compared with previously reported alpha-amylases in the insect hosts of the parasite.