Bacteriocins, defined as ribosomally synthesized antimicrobial peptides, have traditionally been used as food preservatives, either added or produced by starter cultures during fermentation. In-depth studies of a select few bacteriocins opened exiting new research fields and broadened the application of these antimicrobial peptides. The possibility of developing bacteriocins into next generation antibiotics, accompanied with the rapid development in genetics and nanotechnology, paves the way to even more fascinating applications such as novel carrier molecules (delivery systems) and the treatment of cancer. Also, some bacteriocins are found to regulate quorum sensing which suggests novel applications for this group of substances. While there is some interesting translational research on bacteriocins from Gram-negative bacteria, the majority of application-oriented studies are focused on bacteriocins from Gram-positive microorganisms, mostly lactic acid bacteria. The applications of bacteriocins are expanding from food to human health.

Biofilm-associated bacteria are less sensitive to antibiotics than freeliving (planktonic) cells. Furthermore, with variations in the concentration of antibiotics throughout a biofilm, microbial cells are often exposed to levels below inhibitory concentrations and may develop resistance. This, as well as the irresponsible use of antibiotics, leads to the selection of pathogens that are difficult to eradicate. The Centers for Disease Control and Prevention use the terms "antibiotic" and "antimicrobial agent" interchangeably. However, a clear distinction between these two terms is required for the purpose of this assessment. Therefore, we define "antibiotics" as pharmaceutically formulated and medically administered substances and "antimicrobials" as a broad category of substances which are not regulated as drugs. This comprehensive minireview evaluates the effect of natural antimicrobials on pathogens in biofilms when used instead of, or in combination with, commonly prescribed antibiotics.

The intestinal barrier, consisting of the vascular endothelium, epithelial cell lining, and mucus layer, covers a surface of about 400 m2. The integrity of the gut wall is sustained by transcellular proteins forming tight junctions between the epithelial cells. Protected by three layers of mucin, the gut wall forms a non-permeable barrier, keeping digestive enzymes and microorganisms within the luminal space, separate from the blood stream. Microorganisms colonizing the gut may produce bacteriocins in an attempt to outcompete pathogens. Production of bacteriocins in a harsh and complex environment such as the gastro-intestinal tract (GIT) may be below minimal inhibitory concentration (MIC) levels. At such low levels, the stability of bacteriocins may be compromised. Despite this, most bacteria in the gut have the ability to produce bacteriocins, distributed throughout the GIT. With most antimicrobial studies being performed in vitro, we know little about the migration of bacteriocins across epithelial barriers. The behavior of bacteriocins in the GIT is studied ex vivo, using models, flow cells, or membranes resembling the gut wall. Furthermore, little is known about the effect bacteriocins have on the immune system. It is generally believed that the peptides will be destroyed by macrophages once they cross the gut wall. Studies done on the survival of neurotherapeutic peptides and their crossing of the brain–blood barrier, along with other studies on small peptides intravenously injected, may provide some answers. In this review, the stability of bacteriocins in the GIT, their effect on gut epithelial cells, and their ability to cross epithelial cells are discussed. These are important questions to address in the light of recent papers advocating the use of bacteriocins as possible alternatives to, or used in combination with, antibiotics.

Reclassification ofThe type strain of Lactobacillus casei subsp. casei (ATCC 393) exhibits low levels of DNA homology with other strains of L. casei subsp. casei (8 to 46%) and strains of Lactobacillusparacasei (30 to 50%), but exhibits a level of DNA similarity of 80% with Lactobacillus rhamnosus ATCC 15820, the original type strain of "Lactobacterium zeae" Kuznetsov 1959. Strains ATCC 393T (T = type strain) and ATCC 15820T are members of one protein profile cluster that is separate from the other Lactobacillus spp. The randomly amplified polymorphic DNA PCR profile of strain ATCC 393T is also different from the profiles obtained for the other species. L. casei ATCC 334T is genetically closely related to L. casei subsp. casei strains (71 to 97%) and L. paracasei strains (71 to 91%), is a member of the same protein profile cluster as these organisms, and shares several DNA amplicons with L. puracasei strains. On the basis of these results, we propose that L. casei subsp. casei ATCC 393T and L. rhamnusus ATCC 15820 should be reclassified as members of Lactobacillus zeae nom. rev. (type strain, ATCC 15820), that strain ATCC 334 should be designated the neotype strain of L. casei subsp. casei, and that the name L. puracasei should be rejected.

Our world is now facing a multitude of novel infectious diseases. Bacterial infections are treated with antibiotics, albeit with increasing difficulty as many of the more common causes of infection have now developed broad spectrum antimicrobial resistance. However, there is now an even greater challenge from both old and new viruses capable of causing respiratory, enteric, and urogenital infections. Reports of viruses resistant to frontline therapeutic drugs are steadily increasing and there is an urgent need to develop novel antiviral agents. Although this all makes sense, it seems rather strange that relatively little attention has been given to the antiviral capabilities of probiotics. Over the years, beneficial strains of lactic acid bacteria (LAB) have been successfully used to treat gastrointestinal, oral, and vaginal infections, and some can also effect a reduction in serum cholesterol levels. Some probiotics prevent gastrointestinal dysbiosis and, by doing so, reduce the risk of developing secondary infections. Other probiotics exhibit anti-tumor and immunomodulating properties, and in some studies, antiviral activities have been reported for probiotic bacteria and/or their metabolites. Unfortunately, the mechanistic basis of the observed beneficial effects of probiotics in countering viral infections is sometimes unclear. Interestingly, in COVID-19 patients, a clear decrease has been observed in cell numbers of Lactobacillus and Bifidobacterium spp., both of which are common sources of intestinal probiotics. The present review, specifically motivated by the need to implement effective new counters to SARS-CoV-2, focusses attention on viruses capable of co-infecting humans and other animals and specifically explores the potential of probiotic bacteria and their metabolites to intervene with the process of virus infection. The goal is to help to provide a more informed background for the planning of future probiotic-based antiviral research.