# Nisin E, a New Nisin Variant Produced by _Streptococcus equinus_ MDC1
## 1 Introduction  
Bactericons are ribosomally synthesized proteinaceous substances produced by prokaryotes that usually kill or inhibit species or strains closely related to the producer organism [1]. Whilst some bacteriocins conform to this original definition, i.e., have a fairly narrow inhibitory spectrum, many bacteriocins exhibit broad inhibitory spectra and, as a result, are potentially useful as biocontrol agents in the food, agricultural and pharmaceutical industries [2, 3]. Indeed, nisin (produced by _Lactococcus lactis_), one of the first bacteriocins to be studied extensively, has been used as a food preservative for several decades [3]. To date, the majority of known bacteriocins are produced by Gram-positive bacteria and these bacteriocins are generally divided into four classes [1]: (i) lantibiotics, which arepost-translationally modified peptides containing amino acid derivatives such as lanthionine, (ii) small (<10 kilodalton (kDa)) unmodified peptides, (iii) large (>10 kDa) heat-labile proteins and (iv) circular (cyclic) bacterioxins.  
Members of the genus _Streptococcus_ can be found inhabiting a wide variety of habitats in humans and other animals [4]. The streptococci are also prolific producers of a wide range of bacterioxins spanning all four classes, e.g., the lantibiotic salivicrins [5; 6], the non-lantibiotic mutacins [7], the Class III dysgalciticin [8] and the unusual cyclic peptide uberolysin [9]. Furthermore, several streptococcal species have been shown to produce natural variants of nisin (the prototype lantibiotic), e.g., nisin U (_Streptococcus uberis_), nisin H (_Streptococcus hyointestinalis_), nisin P (_Streptococcus agalaciae_) and suicin 90-1330 (_Streptococcus suis_) [10; 11; 12; 13].  
Although streptococci have been isolated from livestock (e.g., cattle and pigs), horses and domestic pets, primarily from dogs, much less is known about streptococci residing in other types of pets such as rabbits and guinea pigs, i.e., does the host range of the genus _Streptococcus_ extend to the latter animal species? Furthermore, to the best of our knowledge, there are no reports of bacteriocin-producing streptococci originating from domestic pets (including dogs and cats), i.e., are there novel bacterioxins to be found in our domestic pets? With these gaps in the literature in mind, we conducted a science fair project to investigate (a) whether streptococci could be isolated from the oral cavities of domestic pets and (b) whether any streptococcal isolates produced bacteriocin-like inhibitory substances (BLIS) [14]. In order to accomplish this, and for sampling convenience, we sampled the oral cavities of five family pets (two guinea pigs, a rabbit, a cat and a dog), using Mitis-Salivarius agar as the isolation medium as it is selective for streptococci. We subsequently tested for antibacterial activity, employing a simultaneous antagonism method [6] against _Microococcus luteus_ T-18, an organism exquisitely sensitive to lantibiotic activity. Not only were presumptive streptococci isolated from all the domestic pets sampled (data not shown) but several \(\alpha\)-hemolytic streptococcal colonies (isolated from the dog) exhibited potent BLIS activity against \(M\). _Iuteus_[14]. One particular BLIS-positive isolate, MDC1, identified as _Streptococcus equinus_ by 16S ribosomal RNA gene sequencing, was selected for further study.  
The aim of the present study is to expand the scope of the original science fair project by (i) investigating the BLIS activity produced by _S. equinus_ strain MDC1, particularly its inhibitory activity spectrum and (ii) identifying the genetic locus specifying the biosynthesis of the putative bacteriocin.