REVIEWS SERIES ON ANTIBIOTIC ALTERNATIVES Bacteriocins — a viable alternative to antibiotics? Paul D. Cotter1,2, R. Paul Ross1,2 and Colin Hill2,3 Abstract | Solutions are urgently required for the growing number of infections caused by antibiotic-resistant bacteria. Bacteriocins, which are antimicrobial peptides produced by certain bacteria, might warrant serious consideration as alternatives to traditional antibiotics. These molecules exhibit significant potency against other bacteria (including antibiotic- resistant strains), are stable and can have narrow or broad activity spectra. Bacteriocins can even be produced in situ in the gut by probiotic bacteria to combat intestinal infections. Although the application of specific bacteriocins might be curtailed by the development of resistance, an understanding of the mechanisms by which such resistance could emerge will enable researchers to develop strategies to minimize this potential problem. Probiotics It could be argued that the identification and development modifications (class I) and unmodified peptides (class II) Live microorganisms that of antibiotic therapy represents the most significant scien- (BOX 1; TABLE 1). Many bacteriocins have a high specific confer a health benefit on the tific achievement of the twentieth century in terms of an activity against clinical targets (including antibiotic- host when administered in impact on human morbidity and mortality. Unfortunately, resistant strains), have mechanisms of action that are dis- adequate amounts. several problems have arisen that limit these initial ben- tinct from current chemotherapeutic products and, given efits and cast doubt on how useful antibiotics will prove their proteinaceous nature, are amenable to gene-based to be in the twenty-first century. Pathogens have emerged peptide engineering. Although several broad-spectrum that are resistant to single, and subsequently multiple, bacterio­cins exist that can be used to target infections of antibiotics. Moreover, there is a shortage of new fami- unknown aetiology, potent narrow-spectrum bacteriocins lies of antibiotics that could potentially compensate for have also been identified that can control targeted patho- resistance to existing antibiotics, in part owing to the high gens without negatively affecting commensal popula- costs and risks associated with developing and using such tions12,13. It should be noted that the emergence of resistant products1,2. Finally, it has become clear that the admin- bacteria is still a possibility, albeit one that might be mini- istration of broad-spectrum antibiotics can lead to ‘col- mized through a detailed understanding of bacteriocin lateral damage’ to the human commensal microbiota, mechanisms of action and through peptide engineering. which has several key roles in host health3–5. The potential In this Review, we highlight the key traits of bacte- association between the use of broad-spectrum antibiotics riocins which suggest that these compounds represent and the increasing incidence of atopic and autoimmune potential alternatives to antibiotics. When using the term diseases is a particular cause for concern3,4. bacteriocins, we refer specifically to small peptide anti- As a consequence, there is a need for the development microbials and thus do not discuss the clinical potential 1Teagasc Food Research Centre, Moorepark, Fermoy, of new antimicrobials that can be used in clinical settings. of larger proteins such as the bacteriolysins, colicins and Cork, Ireland. Alternatives that have been investigated include plant- pyocins. Bacteriocins have also been investigated with 2Alimentary Pharmabiotic derived compounds6, bacteriophages and phage lysins7, respect to their potential in animal health14, in marine Centre, Cork, Ireland. RNA-based therapeutics8, probiotics9, and antimicrobial environments15 and in enhancing food safety and quality11, 3 Microbiology Department, 10 University College Cork, peptides from a variety of sources . One option that but these applications have been reviewed previously College Road, Cork, Ireland. can no longer be ignored is a subgroup of antimicrobial and are not addressed here. e‑mails: peptides known as bacteriocins. These are small, bacteri- [email protected]; ally produced, ribosomally synthesized peptides that are Benefits of bacteriocins [email protected]; active against other bacteria and against which the pro- Bacteriocins have many properties which suggest that [email protected] 11 doi:10.1038/nrmicro2937 ducer has a specific immunity mechanism . Bacteriocins they are viable alternatives to antibiotics. These include Published online are a hetero­geneous group and are usually classified into their potency (as determined in vitro and in vivo), 24 December 2012 peptides that undergo significant post-translational their low toxicity, the availability of both broad- and NATURE REVIEWS | MICROBIOLOGY VOLUME 11 | FEBRUARY 2013 | 95 © 2013 Macmillan Publishers Limited. All rights reserved REVIEWS Box 1 | Classification of bacteriocins been modified to create LFF571 (Novartis) and exhibits potent activity against C. difficile and most other Gram- Several approaches have been taken to classify bacteriocins. One, which is used to positive organisms (with the exception of bifidobacteria classify the bacteriocins of lactic acid bacteria (LAB), divides these peptides into class I and lactobacilli)21. Among the other modified bacte- peptides, which undergo post-translational modification, and class II peptides, which riocins, the sactibiotic thuricin CD has been found to are largely unmodified (or undergo modest modification; for example, the formation of 12 disulphide bridges, circularization or the addition of N‑formylmethionine)11. This system be particularly potent against C. difficile , and another proposes that larger proteins be removed from the bacteriocin category. Similarly, the sactibiotic, subtilosin A, displays a narrow spectrum of antimicrobials that are ribosomally synthesized by Gram-negative organisms can be activity against Enterococcus faecalis, Streptococcus pyo- divided into small peptides, such as microcins, and larger proteins, such as colicins. The genes and Listeria monocytogenes22, as well as Gardnerella microcins have previously been divided on the basis of the presence (class I) or absence vaginalis23. Although the glycocin sublancin 168 does (class II) of significant modification150. We argue that the designation bacteriocin should not seem potent enough to justify commercial applica- be retained for peptide antimicrobials, and thus, ribosomally synthesized antimicrobial tions24, it may be that other glycocins will be identified proteins are not covered in this Review. or created that show greater potential. The proteu- In addition to subdividing bacteriocins on the basis of their modifications, further sin polytheonamide A has been found to be active at subdivisions exist. In the case of modified bacteriocins, a comprehensive nomenclature microgram-per-millilitre concentrations against some, for ribosomally synthesized, post-translationally modified peptides (RiPPs) has recently 25 been proposed151. Given that the RiPPs include modified bacteriocins, this nomenclature albeit non-pathogenic, Gram-positive strains , whereas is also adopted here. Thus, class I (modified) bacteriocins can be subgrouped as bottromycin A2 exhibits potent activity against MRSA 26 lantibiotics152, linaridins118, proteusins25, linear azole- or azoline‑containing peptides153, and VRE . cyanobactins (including patellamide-like and prenylated, anacyclamide-like There are also many examples of unmodified, class II cyanobactins)67, thiopeptides154, lasso peptides155, sactibiotics156, bottromycins157, bacteriocins with potent antimicrobial activity against glycocins158,159, and modified microcins that do not belong to other subgroups (for Gram-positive targets. This includes some class IIa bac- example, microcin C7-C51)160 (TABLE 1). In some of these families, peptides with teriocins, which are active against L. monocytogenes27 antimicrobial activity are rare (linardins) or not well characterized (cyanobactins). and other Gram-positive pathogens28. Class IIc peptides The unmodified or circular (class II) bacteriocins can be divided into five groups that such as the enterococcal bacteriocin (enterocin) AS‑48 correspond to the four subclasses of unmodified LAB bacteriocins and one of the have been investigated predominantly with a view to use subclasses of unmodified microcins. These subclasses are peptides that contain a YGNGV motif (in which N represents any amino acid; the class IIa peptides); two-peptide in non-clinical applications, but do nonetheless possess bacteriocins (class IIb peptides); circular bacteriocins (class IIc peptides); unmodified, antimicrobial activities, which suggests that other appli- 29 linear, non-pediocin-like, single-peptide bacteriocins that do not belong to other cations might be worth pursuing . Several class IId subclasses (class IId peptides); and the microcin E492‑like bacteriocins (class IIe peptides; bacteriocins — for example, epidermicin NI01 — have formerly known as the class IIb microcins). We propose that subclass IIc should be also provided interesting in vitro results30. expanded to include the unmodified anacyclamides161 and that subclass IId should Although there are many examples of bacteriocins incorporate certain microcins, such as microcin V and microcin S89,150. Although with substantial activity against Gram-negative bacte- subclass