Fate of plasmid-bearing, luciferase marker gene tagged bacteria after feeding to the soil microarthropod Onychiurus fimatus (Collembola)

Diversity and metabolic potential of the microbiota associated with a soil arthropod

Springtails are important members of the soil fauna and play a key role in plant litter decomposition, for example through stimulation of the microbial activity. However, their interaction with soil microorganisms remains poorly understood and it is unclear which microorganisms are associated to the springtail (endo) microbiota. Therefore, we assessed the structure of the microbiota of the springtail Orchesella cincta (L.) using 16S rRNA gene amplicon sequencing. Individuals were sampled across sites in the field and the microbiota and in particular the endomicrobiota were investigated. The microbiota was dominated by the families of Rickettsiaceae, Enterobacteriaceae and Comamonadaceae and at the genus level the most abundant genera included Rickettsia, Chryseobacterium, Pseudomonas, and Stenotrophomonas. Microbial communities were distinct for the interior of the springtails for measures of community diversity and exhibited structure according to collection sites. Functional analysis of the springtail bacterial community suggests that abundant members of the microbiota may be associated with metabolism including decomposition processes. Together these results add to the understanding of the microbiota of springtails and interaction with soil microorganisms including their putative functional roles.

Diversity of bacteria associated with Collembola - a cultivation-independent survey based on PCR-amplified 16S rRNA genes

The bacterial communities found in eight different soil-inhabiting microarthropod species of the Class Collembola (springtails) were analyzed by analysis of PCR-amplified 16S rRNA genes obtained without cultivation from total DNA. To characterize the bacteria associated with the parthenogenetically-reproducing Folsomia candida, the almost complete 16S rRNA genes were amplified with three universal primers, i.e., the forward primer F27 and the reverse primer R1492 or R1525. With the reverse primer R1492, 100% of 49 cloned PCR products were identical to the 16S rRNA gene of the intracellular reproduction parasite Wolbachia pipientis (Supergroup E). However, no Wolbachia was detected with the reverse primer R1525 when two different breeding stocks of F. candida were analyzed. Clone libraries from one breeding stock were composed exclusively of a sequence related closely to intracellular bacteria of the genus Rickettsiella (Gammaproteobacteria) (93 clones). In contrast, this sequence was not detected in the other F. candida breeding stock. Instead, sequences of 95 clones originated from different phylogenetic groups (Alphaproteobacteria, Gammaproteobacteria, Firmicutes and Planctomycetes). Several were closely related to bacteria, which are known to live in soil and interact with insects. Genetic profiles based on PCR-amplified partial 16S rRNA genes and single-strand conformation polymorphism (SSCP) indicated that different Collembola species harbored different bacterial communities. SSCP bands indicating Wolbachia pipientis supergroup E were detected in profiles from four of the five parthenogenetic species included in this study. Other dominant bands in the SSCP profiles were related to bacteria from the phyla Proteobacteria, Actinobacteria, Firmicutes and Bacteroidetes. One sequence (AJ605704) indicated the presence of a not-yet-described intracellular symbiont from the Gammaproteobacteria, but several other sequences may represent less specific interactions between environmental bacteria and Collembola.

Conjugative transfer of plasmid-located antibiotic resistance genes within the gastrointestinal tract of lesser mealworm larvae, Alphitobius diaperinus (Coleoptera: Tenebrionidae)

The frequency of conjugative transfer of antimicrobial resistance plasmids between bacteria within the gastrointestinal tract of lesser mealworm larvae, a prevalent pest in poultry production facilities, was determined. Lesser mealworm larvae were exposed to a negative bacterial control, a donor Salmonella enterica serotype Newport strain, a recipient Escherichia coli, or both donor and recipient to examine horizontal gene transfer of plasmids. Horizontal gene transfer was validated post external disinfection, via a combination of selective culturing, testing of indole production by spot test, characterization of incompatibility plasmids by polymerase chain reaction, and profiling antibiotic susceptibility by a minimum inhibitory concentration (MIC) assay. Transconjugants were produced in all larvae exposed to both donor and recipient bacteria at frequencies comparable to control in vitro filter mating conjugation studies run concurrently. Transconjugants displayed resistance to seven antibiotics in our MIC panel and, when characterized for incompatibility plasmids, were positive for the N replicon and negative for the A/C replicon. The transconjugants did not display resistance to expanded-spectrum cephalosporins, which were associated with the A/C plasmid. This study demonstrates that lesser mealworm larvae, which infest poultry litter, are capable of supporting the horizontal transfer of antibiotic resistance genes and that this exchange can occur within their gastrointestinal tract and between different species of bacteria under laboratory conditions. This information is essential to science-based risk assessments of industrial antibiotic usage and its impact on animal and human health.

Symbiotic microorganisms: untapped resources for insect pest control

Symbiotic microorganisms offer one route to meet the anticipated heightened demand for novel insect pest management strategies created by growing human populations and global climate change. Two approaches have particular potential: the disruption of microbial symbionts required by insect pests, and manipulation of microorganisms with major impacts on insect traits contributing to their pest status (e.g. capacity to vector diseases, natural enemy resistance). Specific research priorities addressed in this article include identification of molecular targets against which highly specific antagonists can be designed or discovered, and management strategies to manipulate the incidence and properties of facultative microorganisms that influence insect pest traits. Collaboration with practitioners in pest management will ensure that the research agenda is married to agricultural and public health needs.

Population dynamics and gene transfer in genetically modified bacteria in a model microcosm

The horizontal transfer and effects on host fitness of a neutral gene cassette inserted into three different genomic loci of a plant-colonizing pseudomonad was assessed in a model ecosystem. The KX reporter cassette (kanamycin resistance, aph, and catechol 2, 3, dioxygenase, xylE) was introduced on the disarmed transposon mini-Tn5 into: (I) the chromosome of a spontaneous rifampicin resistant mutant Pseudomonas fluorescens SBW25R; (II) the chromosome of SBW25R in the presence of a naturally occurring lysogenic-phage (phage Phi101); and (III) a naturally occurring plasmid pQBR11 (330 kbp, tra+, Hgr) introduced into SBW25R. These bacteria were applied to Stellaria media (chickweed) plants as seed dressings [c. 5 x 104 colony-forming units (cfu)/seed] and the seedlings planted in 16 microcosm chambers containing model plant and animal communities. Gene transfer to pseudomonads in the phyllosphere and rhizosphere was found only in the plasmid treatment (III). Bacteria in the phage treatment (II) initially declined in density and free phage was detected, but populations partly recovered as the plants matured. Surprisingly, bacteria in the chromosome insertion treatment (I) consistently achieved higher population densities than the unmanipulated control and other treatments. Plasmids were acquired from indigenous bacterial populations in the control and chromosome insertion treatments. Plasmid acquisition, plasmid transfer from inocula and selection for plasmid carrying inocula coincided with plant maturation.

Protocol for rapid fluorescence in situ hybridization of bacteria in cryosections of microarthropods

A protocol was developed to detect bacteria inhabiting microarthropods by means of small-subunit rRNA-targeted fluorescence in situ hybridization and microscopy. The protocol is based on cryosections of whole specimens. In contrast to more commonly applied paraffin-embedding techniques, the protocol is quicker and reduces the number of manipulations which might damage the microscopic material. The method allowed the study of the bacterial colonization of Folsomia candida (Collembola) and the detection of bacteria in both the gut and tissue.

Intergeneric transfer of chromosomal and conjugative plasmid genes between Ralstonia solanacearum and Acinetobacter sp. BD413

Conjugative transfer of a broad-host range plasmid and transformation-mediated transfer of chromosomal genes were found to occur at significant frequencies between Ralstonia solanacearum and Acinetobacter sp. in planta. These intergeneric gene transfers are related to the conditions provided by the infected plant, including the extensive multiplication of these two bacteria in planta and the development of a competence state in Acinetobacter sp. Although interkingdom DNA transfer from nuclear transgenic plants to these bacteria was not detectable, plants infected by pathogens (e.g., Ralstonia solanacearum) and co-colonized by soil saprophyte bacteria (e.g., Acinetobacter sp.) can be considered as potential "hot spots" for gene transfer, even between phylogenetically remote organisms.

Contribution of the Earthworm Lumbricus rubellus (Annelida, Oligochaeta) to the Establishment of Plasmids in Soil Bacterial Communities

The contribution of the earthworm Lumbricus rubellus in spreading plasmids from a nonindigenous bacterial species to the soil microbial community was studied with Escherichia coli strains as donor organisms. The selected donor strains harbored marker-gene tagged plasmids with different transfer properties and host ranges. Prototrophic benzoate degrading indigenous bacteria were analyzed as potential recipients. In filter-mating experiments, donor strains were mixed with bacterial cell consortia extracted from earthworm casts (feces) and incubated on nutrient agar at 28 degrees C. Transfer was detected with the broad host range IncP plasmid pRP4luc; with the IncQ plasmid, pSUP104luc, but only when it was present in a mobilizing donor strain; and with the transposon delivery vector pUTlux. No transfer was detected with the nonmobilizable pUCluc and the mobilizable pSUP202luc, both of narrow host range. In microcosm studies with E. coli inoculated soil incubated at 12 degrees C, transconjugants were only detected in casts of L. rubellus but not in bulk soil, indicating that the gut passage was a precondition for plasmid transfer. Plasmid pRP4luc was transferred at higher frequencies than detected in filter mating. Results of the filter matings were confirmed except that transfer of pUTlux could not be detected. The majority of transconjugants isolated in this study lost their acquired plasmid upon further cultivation. Stable transconjugants, however, were obtained and identified at the 16S rRNA gene level as members of the b- and g-subgroups of Proteobacteria. Incubation of E. coli and selected transconjugants in soil microcosms with L. rubellus demonstrated that the gut passage resulted in a slight but significant reduction of ingested cells. In contrast to the donor strains, however, the population sizes of transconjugants in bulk soil and in casts did not decrease over time. This demonstrated that the transferred plasmids had established themselves in the soil microbial community.