ULTRASTRUCTURE OF THE AGENT OF CITRUS "STUBBORN" DISEASE

Bacterial diversity in Solenopsis invicta and Solenopsis geminata ant colonies characterized by 16S amplicon 454 pyrosequencing

Social insects harbor diverse assemblages of bacterial microbes, which may play a crucial role in the success or failure of biological invasions. The invasive fire ant Solenopsis invicta (Formicidae, Hymenoptera) is a model system for understanding the dynamics of invasive social insects and their biological control. However, little is known about microbes as biotic factors influencing the success or failure of ant invasions. This pilot study is the first attempt to characterize and compare microbial communities associated with the introduced S. invicta and the native Solenopsis geminata in the USA. Using 16S amplicon 454 pyrosequencing, bacterial communities of workers, brood, and soil from nest walls were compared between neighboring S. invicta and S. geminata colonies at Brackenridge Field Laboratory, Austin, Texas, with the aim of identifying potential pathogenic, commensal, or mutualistic microbial associates. Two samples of S. geminata workers showed high counts of Spiroplasma bacteria, a known pathogen or mutualist of other insects. A subsequent analysis using PCR and sequencing confirmed the presence of Spiroplasma in additional colonies of both Solenopsis species. Wolbachia was found in one alate sample of S. geminata, while one brood sample of S. invicta had a high count of Lactococcus. As expected, ant samples from both species showed much lower microbial diversity than the surrounding soil. Both ant species had similar overall bacterial diversities, although little overlap in specific microbes. To properly characterize a single bacterial community associated with a Solenopsis ant sample, rarefaction analyses indicate that it is necessary to obtain 5,000-10,000 sequences. Overall, 16S amplicon 454 pyrosequencing appears to be a cost-effective approach to screen whole microbial diversity associated with invasive ant species.

A proposed life cycle model of Spiroplasma mirum based on scanning electron microscopical observations of growth in liquid culture

Cells of Spiroplasma mirum strain SMCA were grown in PPG broth and examined by scanning electron microscopy. Comparison of the results at different time intervals allowed for a model of the life cycle of S. mirum to be proposed. Under favorable growth conditions, helical filament formation was initially observed, followed by the formation of small spherical structures originating from each filament. In old culture, large spherical bodies appeared from entangled helical filaments. From the larger spherical bodies, granular bodies representing the smallest reproductive units were produced to continue the life cycle.

Mycoplasma viruses

Unlike bacterial viruses that infect cells bounded by a cell wall, mycoplasma viruses have evolved to enter and propagate in mycoplasma cells bounded only by a single lipid-protein cell membrane. In addition, mycoplasmas have the smallest amount of genetic information of any known cells, so their complexity is constrained by a limited genetic coding capacity. As a consequence of these host cell differences, mycoplasma viruses have been found to have a variety of structures and replication strategies which are different from those of the bacterial viruses. This article is a critical review of mycoplasma viruses infecting the genera Acholeplasma, Spiroplasma, and Mycoplasma; included are data on classification, morphology and structure, biological and physical properties, chemical composition, and productive and lysogenic replication cycles.

Spiroplasmas and acholeplasmas: multiplication in insects

The helical wall-free microorganism, Spiroplasma citri, which is associated with citrus stubborn, a disease with no known vector, multiplied in the leafhopper vector of corn stunt but multiplied to higher titer in the vector of aster yellows and decreased the longevity of that insect. Acholeplasma laidlawii and A. granularum also multiplied in both leafhoppers.