Microplastics as a Novel Mode of Transportation for Potentially Dangerous ‘Hitchhiking’ Vibrio Spp.

 As the rate of global plastic production continues to increase, so too does public awareness of plastic pollution within the marine environment. Whilst the hazards surrounding larger plastic debris have received considerable attention from researchers, relatively little is known about the threats posed by much tinier ‘microplastics’ and how they might interact with marine microbial communities.

 Microplastics are generally classified as plastic fragments less than 5mm in diameter and occur when larger plastics break down due to weathering. As with larger plastic debris, microplastics in the marine environment become swiftly colonised by a mosaic of bacteria and other microscopic organisms, conglomerating to form complex biofilms. Previous studies have highlighted how marine plastic debris-associated microbial populations usually differ in composition from the surrounding bacterioplankton. As such, the term ‘Plastisphere’ has arisen, recognising plastic surfaces as novel habitats for their microbial constituents.

 Being lightweight and small in size, microplastics are suspended and transported readily by waves and currents, allowing their microbial colonisers to hitch rides over considerable distances. Accordingly, it is plausible that microplastics may serve as novel vectors for the transportation and dispersal of microbes.

 One recent study performed by Kirstein et al. investigated the occurrence of potentially human pathogenic Vibrio bacteria colonising microplastics within the North and Baltic Seas. The primary focus of the study was to detect the presence of V. cholerae, V. parahaemolyticus and V. vulnificus; known to cause disease in humans.

 Microplastic particles were collected from surface waters using a fine mesh net and the surrounding surface seawater was also sampled so that the occurrence of Vibrio spp. could be compared between microplastic-attached and bacterioplankton populations. Microplastic and filtered seawater samples were incubated individually at 37°C to allow for the selective enrichment of mesophilic Vibrio spp. Subsequently, samples displaying growth were plated onto a vibrio-selective agar media and incubated further. Resulting colonies resembling V. cholerae, V. parahaemolyticus and V. vulnificus were analysed via MALDI-TOF mass spectrometry and PCR amplification of species-specific genes in order to confirm their identification.

 Of the collected microplastics analysed within this study, 13% were found to exhibit colonisation by cultivatable Vibrio spp. Whilst V. cholerae and V. vulnificus were detected only within seawater samples, potentially pathogenic V. parahaemolyticus was isolated from a number of microplastic particles. Where V. parahaemolyticus was detected, it was generally present both attached to microplastics and within the surrounding seawater, suggesting that colonisation of microplastics by Vibrio spp. may occur from the surrounding bacterioplankton. Furthermore, sampling efforts within the Baltic Sea were unable to detect V. parahaemolyticus within surface waters yet identified it associated to a single microplastic particle. Such an observation may serve as an illustration of Vibrio spp. utilising microplastic to 'hitch' a ride, persisting into an environment where it may have otherwise been absent.

 Ultimately, Kirstein et al. have made an important contribution to the understanding of the hazards surrounding microplastic pollution by providing the first definitive proof of potentially human pathogenic Vibrio spp. within marine microplastic biofilms. As average sea surface temperatures continue to rise, the occurrence of Vibrio infection is increasing in frequency and geographical range. Moreover, as larger plastic items repeatedly fragment within the natural environment, marine microplastic concentrations are set to increase. Accordingly, global warming coupled with the emerging problem of marine microplastic could prove to be a perilous combination. In order to fully comprehend the hazards posed by microplastic-attached Vibrio spp., subsequent research could perhaps employ a metagenomic approach in order to gain a clearer understanding of Vibrio abundance, avoiding possible cultivation bias.              

Reviewed Paper:

Kirstein, I. V., Kirmizi, S., Wichels, A., Garin-Fernandez, A., Erler, R., Löder, M. & Gerdts, G. (2016). Dangerous hitchhikers? Evidence for potentially pathogenic Vibrio spp. on microplastic particles. Marine Environmental Research, 120: 1-8.