UC Riverside

Recent studies have elucidated host health implications of the bumble bee and honey bee (Family: Apidae) microbiome. However, less is known wild bee microbiomes, or microbiomes of non-corbiculate apids. Lactobacillus micheneri and its close relatives, L. timberlakei and L. quenuiae consistently associate with wild bees. Through bacterial genome sequencing, population genomics, bacterial 16S rRNA gene sequencing, culturing, and quantitative PCR, I studied the genetic capabilities, transmission mechanisms, and ecological interactions of the three lactobacilli species I refer to as the L. micheneri clade.

Bacterial genomes reveal potential functions while population and comparative genomics can identify important genes under selection to their hosts and environment. I sequenced 27 genomes from the L. micheneri clade and compared them to closely related, free-living and insect-associated Lactobacillus. I found that the L. micheneri clade has traits that imply adaptation towards the bees and flowers.

Transmission modes are important in maintaining microbial interactions between generations of hosts. To investigate transmission mechanisms of Lactobacillus micheneri to bees, I sequenced bacterial 16S rRNA gene to compare bacterial communities in overwintering leaf cutter bee nests and newly emerged bees. I found that L. micheneri survives over the winter but transmits to the next generation of bees rarely at best, and therefore appears to be preserved and transmitted elsewhere in the environment.

Lactobacilli can inhibit microorganisms in many human foods. Pollen provisions collected by bees are nutrient dense yet are normally not colonized with saprophytic fungi. Instead, Lactobacillus micheneri are abundant inside pollen provisions. I tested L. micheneri fungal inhibitory activity on plates and in sterilized pollen provisions. I found that while most L. micheneri strains can inhibit fungi, L. timberlakei is a strong fungal inhibitor on plates and in pollen provisions.

In this dissertation, I have contributed new knowledge and demonstrated that Lactobacillus micheneri is an interesting insect-associated symbiont. There are signatures of adaptation towards bees and flowers, and Lactobacillus are proven to inhibit microorganisms to the benefit of bees. However, while wild bee larvae benefit from L. micheneri’s ability to inhibit fungi, Lactobacillus micheneri does not readily vertically transmit and may be heavily reliant on horizontal transmission.