Research on antimicrobial peptides has been pursued for more than four decades. This field has attracted a great deal of attention, searching among microbial species from virtually any corner of the planet, to find microbes as a potential source of therapeutic agents that can be screened for antimicrobial, antifungal, antiviral, immunomodulating, immunosuppressive, anti-inflammatory and antitumor activities. These small peptides or lantibiotics (<4 kDa) are characterized by their high content of thioether linked amino acid residues and unsaturated dehydrated residues.
The lantibiotics are very active against pathogenic Gram positive bacteria (e.g., B. cereus) and fungi (e.g., A. flavus) that are directly responsible for some food borne illnesses. This class of peptides has the capacity to be used as a biopreservative to protect perishable food products from pathogen contamination, prevent spoilage, inhibit pathogens and prevent infections in humans and animals.
Various microbial species have also been shown to produce a variety of secondary metabolites of interest and utility. Among these are small peptide molecules that represent a very large and diverse subclass of bioactive natural products that have unique structural features involved in the morphology, physiology and survival of the microbe. There are, for example, amphiphilic lipopeptides which are aggressive surface tension reducing agents, such as surfactin and the antifungal mycosubtilin. In these compounds, the lipo-substituent plays a key role in disrupting the cell membrane, while the amphiphilic component exhibits disruptive hemolytic properties. These properties make them very strong antiviral and antibacterial compounds.
Consumption of certain live microorganisms has been shown in some circumstances to have a beneficial impact on man and animals. A diverse group of microbes has been evaluated for such “probiotic” activity, including many species of the genera Lactobacilli and Bifidobacteria. They are the most abundant in probiotic-containing food products. Less commonly, species of Enterococcus, Saccharomyces, non-pathogenic Escherichia, and spore-forming Sporolactobacillus, Brevibacillus and non-pathogenic Bacillus, have been suggested for probiotic foods.
The commensal microflora in the intestine is a complex ecosystem with interactions among host cells, nutrients and microflora. An adult human body contains a living bacterial biomass of greater than 1014 and more than 400 different species. The probiotic bacteria help to keep pathogenic bacteria at bay. Also, “symbiotic” bacteria from industrial and traditional fermented foods may also contribute to the development of a healthy gastrointestinal microflora (e.g., a significant enrichment in the bacterial phylum Bacteriodetes and depletion in the bacterial phylum Firmicutes in the human gut microbiota).
The “hygiene hypothesis” proposed in 1989 by David Strachan correlated lower environmental exposure to microbes—as seen in developed countries—with higher rates of allergies. Western developed countries successfully controlled infectious diseases during the second half of the 20th century by improving sanitation and by using antibiotics and vaccines. At the same time, a rise in new diseases occurred such as allergies, autoimmune disorders, and inflammatory bowel disease (IBS) both in adults and in children. The gastrointestinal microflora are known to play an important role in IBS pathogenesis. Obesity is also associated with an imbalance in the normal gut microbiota.
Corn tortillas are a common staple in the diets of the Mexican, Mexican-American and Central American populations. Traditionally, corn tortillas are prepared by steeping whole-kernel corn overnight in a lime solution (1 to 2% w/w), and subsequently grinding the drained and rinsed and/or peeled grain (now referred to as nixtamal) into a fresh dough called masa (56% moisture content). Natural fermentation and souring in the masa can lead to microbial spoilage, often within 5-12 hours. Commercially, the dough is stacked at ambient temperature and held for a period of time (a few minutes to several hours) before being formed and baked into tortillas (45-50% final moisture).
Commercial tortilla mills (Central Mexico) have been shown to produce corn dough with counts of lactic acid bacteria (i.e., Streptococcus spp. and Lactobacillus spp.) and aerobic mesophiles (i.e., Streptococcus bovis) ranging from 10,000 to 10,000,000 CFU/g. Coliforms were also found from 100 to1000 CFU/g, while fungal counts were <10 CFU/g (Adolphson et al., 2013).
Studies of fermented maize foods in southeastern Mexico found lactic acid bacteria (i.e., Leuconostoc and Lactococcus spp.) present at 100,000-1,000,000 CFU/g in pozol, a fermented nixtamalized corn dough consumed as a beverage mainly by the indigenous Mayan populations (Wacher et al., 2000). A mixed-culture fermentation was typical in pozol and a varied microflora have been isolated (Ulloa et al., 1983; Ramirez, 1987; and Steinkraus, 1996) such as spore forming bacteria (B. cereus and Bacillus spp.), coliform bacteria (E. coli and K. pneumonia), free-living nitrogen fixing bacteria (K. pneumonia and A. pozolis), lactic acid bacteria (Lactobacillus spp.) and yeast and molds (Candida spp., Geotrichum candidum, Monilia sitophila and Penicillium sp.).
The ecology of food fermentation and spoilage includes several factors: (a) intrinsic factors (physico-chemical properties of the food material), (b) microbiological changes due to the modes of processing (endogenous or exogenous microorganisms), (c) extrinsic factors (water activity, oxygen tension and temperature) and (d) implicit factors (symbiosis or antagonism amongst microflora). The North American Millers Association (NAMA) published a survey for the microbiological profile of five dry-milled grains (wheat, corn, oats, whole wheat and durum) showing similar or reduced counts for indicator tests (total aerobes, fungi, coliform and E. coli) (Sperber et al., 2007.
Food processing technologies allow many products to be marketed safely, and the keeping quality depends on the formulation of chemical (inorganic/organic acids) and novel compounds that inhibit microbial growth and control the microbial ecology in the food production environment. Natural and/or biocidal agents include antimicrobial compounds from plants, bacteria and animals, and cultures (e.g. probiotics excreting bacteriocins and lantibiotics).