# Nisin E, a New Nisin Variant Produced by _Streptococcus equinus_ MDC1
## 3 Results  
### S. equinus MDC1 Produces a Nisin U-like Bacteriocin  
When the BLIS activity produced by _Streptococcus equinus_ MDC1 was tested initially against the set of nine standard indicator bacteria [15], it was observed that the growth of _Streptococcus_ _uberis_ ATCC 27958 (standard indicator I4) was not inhibited. As _S. uberis_ \begin{table}
\begin{tabular}{c c} \hline
**Species** & **Strain(s) and/or Description(s)** \\ \hline _Lactococcus lactis_ & T-21 (standard indicator I6), 4862, LM0230, DRC2, 346, MG1614, 166, E8 \\ _Streptococcus dysgallariae_ & T-148 (standard indicator I9) \\ _Streptococcus gordanii_ & NCTC 10558 (type strain), DL1, C219, Wicky, OB143; all human oral isolates \\ _Streptococcus oralis_ & NCTC 10557 (type strain), 34, SK10, SK39, SK616; all human oral isolates \\ _Other S. uberis_ & O140] (genome reference strain), E, Buddle, D528, D536 \\ _Streptococcus anginos_ & NCTC 10713 (type strain), OB98, OB105, all human oral isolates \\ _Streptococcus initis_ & OB638, OB711, OB721, OB722; all human oral isolates \\ _Streptococcus salivarius_ & NCTC 8618, M18, JH \\ _Streptococcus angitis_ & NCTC 10556 (type strain), OB633; both human oral isolates \\ _Streptococcus australis_ & 118; human oral isolate \\ _Streptococcus kieseri_ & CB1; oral isolate from a New Zealand brushtail possum \\ _Abiotrophia defectiva_ & CW1; human oral isolate \\ _Enterococcus faecalis_ & V583; genome reference strain and human clinical isolate \\ _Bacillus provicus_ & M11; environmental isolate \\ _Macrococcus equipercius_ & M21, M23; both environmental isolates \\ _Microoccus alvererae_ & M27, M31, M32, M33, M34, M35, M36; all environmental isolates \\ _Staphylococcus aureus_ & MChE1, S3, 9, S16; all environmental isolates \\ _Escherichia coli_ & DH5c; Gram-negative species \\ \hline \end{tabular}
\end{table} 
Table 1: Cont. ATCC 27958 is a known producer of nisin U (and nisin U alone) [10], it was speculated that strain MDC1 could be producing a nisin-like lantibiotic bacteriocin. For that reason, we decided to compare the inhibitory activity spectra of the two strains. The inhibitory activity of _S. equinus_ MDC1 and _S. uberis_ ATCC 27958 was tested by deferred antagonism assays against >70 indicator strains from a range of sources including oral bacteria (human and animal) and environmental isolates (Table 1). As shown in Table 2, the inhibitory spectra of both strains are largely similar but there are distinct differences. Whereas both bacteriocins were especially potent against strains of _Microoccus_ and _Macroococcus_, _Streptococcus__sanguinis_, and _Streptococcus__pyogenes_, the MDC1 BLIS exhibited a reduced inhibitory spectrum (i.e., by 23 fewer strains), particularly against strains of _Lactococcus lactis_ (5 of 8), _Streptococcus__custellatus_ (3 of 8), _Streptococcus__calis_ (1 of 5), _Streptococcus__gordonii_ (0 of 5), _Streptococcus__anginos_ (0 of 4) and _Streptococcus__salaviarus_ (0 of 3) (Table 2). Furthermore, there were noticeable differences in the potency of inhibitory activity between nisin U and the MDC1 BLIS with strains of _Streptococcus__mitis_ (Table 2). Both bacteriocins do not kill _Staphylococcus aureus_, _Enterococcus faecalis_ and _Escherichia coli_, a finding consistent with previous experiments with nisin U [10]. Interestingly, whilst _S. uberis_ ATCC 27958 was immune to the inhibitory effects of the MDC1 BLIS, the reverse was not true as partial inhibition of MDC1 was observed with nisin U. This unexpected result will be discussed below. Nevertheless, these results collectively support the hypothesis that the MDC1 BLIS is a new natural variant of the nisin family. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**Producer Strain (Bacteriocin Produced)**} \\ \cline{2-5}  & _S. uberis_ ATCC 27958 (Nisin U) & _S. equinus_ **MDC1 (Nisin E)** \\ \hline
**Indicator Bacteria (no. of Strains)** & **No. of Strains** & **Potency of Inhibitory** & **No. of Strains** & **Potency of Inhibitory** \\ \hline _S.