An end to antibiotic resistance??

Vibrio anguillarum is the causative agent of vibrosis, a haemorrhagic septicaemia disease of many commercially important fish. Antibiotics are still the primary means by which bacterial diseases are controlled in aquaculture, despite the increasing prevalence of multi-resistant bacteria. It is essential that novel approaches to tacking antibiotic resistance are developed; failure to do so would be detrimental. Furthermore, typically broad-spectrum antibiotics are used; despite a greater number of pathogenic bacteria being eliminated, this lack of specificity can devastate the commensal bacterial consortia of the host. Whilst vaccines to treat vibrosis exist, these target the O1 and O2 serotypes of V. anguillarum which are most associated with outbreaks of the disease. However this vaccine does not effectively treat vibrosis induced by the O3 serotype, which has increased in incidence in recent years. The use of biological agents such as bacteriophages may be one means by which bacterial infection is controlled. This has been exemplified in a number of studies in varying fields. Higuera et al. (2013) provide the first evidence in support of phage use in treating V. anguillarum infection, in the commercially important species, Salmo salar.

V. anguillarum samples were donated to researchers; these were originally isolated from a number of fish farm. The strains of the species used were determined using 16S rRNA analysis. Phages were isolated from bivalves (mussels and clams); 6 phages (309, ALMED, CHOED, ALME, CHOD and CHOB) were able to infect V. anguillarum, these were characterised using electrophoresis and electron microscopy. 15 S.salar were added to aquaria, and the water was inoculated with V. anguillarum PF4 at a concentration of 5x10⁵ CFU/ml, straight after the CHOED phage was added to the water at a concentration to allow the required multiplicity of infection (MOI) to be obtained. This same experimental system was set up in a fish farm setting, where 100 S. salar were exposed to test conditions. Daily checks of mortality were undertaken and dead individuals were removed then examined for the presence of the pathogenic bacteria.  Regular examination of bacterial and phage concentrations was performed, along with the necessary dilutions.

When added to the water V. anguillarum (5x10⁵ CFU/ml) with no phage present, mortality was observed in the salmon from day one, by day six there was 93% mortality. With addition of the CHOED bacteriophage at a concentration of 1 PFU/bacterial cell, fish survived up to 10 days, post-infection. In control conditions, 70% survival was noted after 10 days post infection; this is not attributed to infection by V. anguillarum however, as no V. anguillarum individuals were noted in or on the fish. When subject to a higher MOI of 20 PFU/bacterial cell, there was a significant decline in the V. anguillarum individuals present (2.9 x10⁴ at day 1 post infection, declining to <1.10³ at day 6). At day 7 no V. anguillarum was detected in the tanks, however phages were present in abundance with 1x10⁶ PFU/cell. However a large decline in the presence of Vibrio was observed even when not subject to bacteriophage infection.  In the fish farm setting, 20 days post-infection (with V. anguillarum alone), 60% of S. salar were alive, however with addition of the CHOED phage at 10 PFU/cell, 100% survival was observed again after 20 days.

This study concludes that phages may be an alternate therapeutic tool to treat bacterial infection. The supporting evidence is not completely convincing; the paper did not state that any of the fish tested had contracted vibrosis, thus whether introduction of phages reduces vibrosis or merely the presence of V. anguillarum remains unanswered. Furthermore, introduction of these phages could have an impact on the microbiota of fish species, this too needs to be elucidated. In the literature, there is some debate about how phages are introduced as this could impact the results obtained. However, novel ideas such as this are vital in order to reduce the occurrence of antibiotic resistance.

Jack 

Reference

Higuera, G., Bastias, R., Tsertsvadze, G., Romero, J. and Espejo, R.T. (2013) Recently discovered Vibrio anguillarum phages can protect against experimentally induced vibrosis in Atlantic salmon, Salmo salar.  Aquaculture, 392 (295), 128-133.