**4. Alternatives to antibiotics**

The fact that the introduction of new antibiotics in the market decreased over the last decades together with the appearance of resistance fueled the investigation of alternative sources of antimicrobial agents.

The new research venues include bacteriocins, phages, and nanoparticles.

#### **4.1 Bacteriocins**

Bacteriocins are short or long sequences of amino acids with antibacterial activities produced by lactic bacteria. Their sequences are heterogeneous and classified according to their molecular weight [28]. For example, some of them consist of short peptide sequences (19–37 amino acids), but others can reach molecular weights of up to 90,000 Da.

Bacteriocins are considered to possess antibacterial activity against a broad spectrum of bacteria, making them nonspecific and considered safe and natural antimicrobial agents because of their consumption in dairy products since ancient times [29]. In other words, bacteria considered beneficial to human produce bacteriocins.

Bacteriocins are produced by lactic bacteria in the intestine probably to gain access to nutrients in a highly competitive environment with trillions of different bacterial species striving to survive. However, bacteriocins are not exclusive to the lactic bacteria group. Other bacterial strains have been shown to produce bacteriocin as well such as *Fusobacterium mortiferum* and *Enterococcus faecium*, which was isolated from chicken with *in vitro* antibacterial activities [30, 31].

Bacteriocins are grouped in different classes, but lantibiotics and thiopeptides are the most extensively studied [32]. For example, lantibiotics are very effective to control Gram-positive infections *in vitro* and *in vivo* caused by *Staphylococcus aureus*, *Staphylococcus epidermidis*, *Streptococcus pneumonia*, and *Streptococcus pyogenes* [33–36].

On the other hand, thiopeptides have shown extraordinary results as antimicrobial agents, but their applications have been restricted because of water solubility issues [37, 38]. However, analogs of these thiopeptides have been generated with successful applications to control infections of *Clostridium difficile*, *Salmonella enterica*, and *Staphylococcus aureus* using rodent models [39–41].

Although bacteriocins can be delivered as bacteriocin-producing bacteria, their activity in the intestinal tract should be monitored. In the case of bacteriocin treatment in chicken, it has been shown that low-molecular-weight bacteriocins are active in the intestinal environment. For instance, the secretion of curvacin produced by *Lactobacillus curvatus* showed growth inhibition of the pathogens *Escherichia coli* and *Listeria innocua* in the digestive tract [42]. Experiments performed to determine the degradation of the bacteriocin in the digestive tract revealed that it was degraded in the last portion of the intestine (ileum) [42].

The bacteriocin nisin produced by *Lactobacillus lactis* showed a change in the fermentation parameters in an artificial rumen model [43]. These changes are probably attributed to changes in the microbiome of the rumen caused by the bacteriocin administration.

#### *4.1.1 Mechanism of action of bacteriocins*

Studies have reported that bacteriocins target different pathways. For example, lantibiotics and other bacteriocins bind the lipid II, which is an intermediate in the peptidoglycan biosynthesis [44–46]. Moreover, upon binding lipid II, lantibiotics enable the formation of pores in the bacterial cell membrane leading to a membrane potential unbalance, resulting in cell death [44, 45].

It looks that pore formation is a mechanism observed in different types of bacteriocins. Their activity depends on the binding to specific receptors on the bacterial membrane in order to exert their activity. For instance, some bacteriocins recognize the cell envelope-associated mannose phosphotransferase system (Man-PTS), whereas others recognize siderophore receptors (e.g., FepA, CirA, or Fiu) [47, 48].

Other mechanisms of action of bacteriocins such as the interference in gene expression and protein biosynthesis have been proposed. Examples include interference with DNA (e.g., inhibition of supercoiling mediated by gyrase), RNA (e.g., blocking mRNA synthesis and binding to the 50S ribosomal subunit), and protein synthesis (e.g., modification of amino acids and binding to the elongation factor Tu) [49–53].

**9**

*DOI: http://dx.doi.org/10.5772/intechopen.83691*

tance to other antibiotics [57, 58].

have been observed [60–62].

*4.1.3 Bacteriocin delivery*

**4.2 Antimicrobial peptides**

antibacterial agents should be evaluated as well.

microbiome for 14 years after the application [67].

conformation change to intercalate in the membrane.

resistant to the activity of proteases [70–73].

*4.2.1 Mechanism of action of antibacterial peptides*

*4.1.2 Mechanisms of resistance to bacteriocins*

The appearance of resistance is always a concern that may be developed as a result of changes in the membrane composition/structure. In this regard, resistance to nisin has been reported in specific strains of *Clostridium* and *Listeria* [54–56]. Moreover, exposure of the bacteriocins microcin-24 and nisin to *Salmonella enterica* and *Streptococcus bovis*, respectively, showed that the resistant cells had also resis-

Resistance mechanisms have been mainly identified with bacteriocins targeting the cell envelope. In this regard, studies have shown that a decrease in the receptor of the bacteriocin targeting the lipid II conferred resistance to *Staphylococcus aureus* [46] and a regulation of the ABC transporter in *Listeria monocytogenes* [59]. However, mutations on genes encoding the RNA polymerase subunit and the gyrase

In conclusion, resistance to bacteriocins has already been reported and potential solutions should be taken into consideration to reduce the appearance of such resistance. These include the derivatization of the original molecule to synthesize new molecules that may bind the receptors to reduce their recognition by the bacteria [63]. Alternatively, the use of a cocktail of bacteriocins in combination with other

One attractive system for the delivery of bacteriocins is the use of *Lactobacillus* strains. For example, growth inhibition of the pathogens *Listeria monocytogenes* and enterohemorrhagic *Escherichia coli* in a mouse model has been reported using *Lactobacillus casei* str. LAFTI L26 [64, 65]. Interestingly, the use of bacteriocins was successful to control buccal pathogens by using an engineered *Streptococcus mutans* strain, which produces the bacteriocin mutacin 1140. This bacteriocin was able to control plaque formation [66] and the engineered strain was retained in the buccal

Small antimicrobial peptides are produced by probably every organism to cope with bacterial invasion. Antimicrobial peptides are short peptides with a molecular mass of 1000–5000 Da. Analysis of their sequences revealed that they interact with the negatively charged of bacterial membranes based on their net positive charge [68]. Further analysis of antibacterial peptides revealed that in their sequences a hydrophobic sequence is required to bind to the bacterial membrane as well as a

Structural analysis of the peptides showed that they may acquire different 3D-conformations such as helices, sheets, or loops [69]. The structure of the peptides is very important because a redesign of the secondary structures of the peptides may increase their antibacterial activities or their stability being more

It appears that the main mechanism of action of antibacterial peptides is permeabilization. Therefore, they depend on the interaction with the cell membrane. This interaction involves an electrostatic interaction when the cationic peptide

*A New Era without Antibiotics*

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bacteriocins.

[33–36].

riocin administration.

*4.1.1 Mechanism of action of bacteriocins*

potential unbalance, resulting in cell death [44, 45].

Bacteriocins are considered to possess antibacterial activity against a broad spectrum of bacteria, making them nonspecific and considered safe and natural antimicrobial agents because of their consumption in dairy products since ancient times [29]. In other words, bacteria considered beneficial to human produce

Bacteriocins are produced by lactic bacteria in the intestine probably to gain access to nutrients in a highly competitive environment with trillions of different bacterial species striving to survive. However, bacteriocins are not exclusive to the lactic bacteria group. Other bacterial strains have been shown to produce bacteriocin as well such as *Fusobacterium mortiferum* and *Enterococcus faecium*, which was

Bacteriocins are grouped in different classes, but lantibiotics and thiopeptides are the most extensively studied [32]. For example, lantibiotics are very effective to control Gram-positive infections *in vitro* and *in vivo* caused by *Staphylococcus aureus*, *Staphylococcus epidermidis*, *Streptococcus pneumonia*, and *Streptococcus pyogenes*

On the other hand, thiopeptides have shown extraordinary results as antimicrobial agents, but their applications have been restricted because of water solubility issues [37, 38]. However, analogs of these thiopeptides have been generated with successful applications to control infections of *Clostridium difficile*, *Salmonella* 

Although bacteriocins can be delivered as bacteriocin-producing bacteria, their activity in the intestinal tract should be monitored. In the case of bacteriocin treatment in chicken, it has been shown that low-molecular-weight bacteriocins are active in the intestinal environment. For instance, the secretion of curvacin produced by *Lactobacillus curvatus* showed growth inhibition of the pathogens *Escherichia coli* and *Listeria innocua* in the digestive tract [42]. Experiments performed to determine the degradation of the bacteriocin in the digestive tract revealed that it was degraded in the last portion of the intestine (ileum) [42]. The bacteriocin nisin produced by *Lactobacillus lactis* showed a change in the fermentation parameters in an artificial rumen model [43]. These changes are probably attributed to changes in the microbiome of the rumen caused by the bacte-

Studies have reported that bacteriocins target different pathways. For example, lantibiotics and other bacteriocins bind the lipid II, which is an intermediate in the peptidoglycan biosynthesis [44–46]. Moreover, upon binding lipid II, lantibiotics enable the formation of pores in the bacterial cell membrane leading to a membrane

It looks that pore formation is a mechanism observed in different types of bacteriocins. Their activity depends on the binding to specific receptors on the bacterial membrane in order to exert their activity. For instance, some bacteriocins recognize the cell envelope-associated mannose phosphotransferase system (Man-PTS), whereas others recognize siderophore receptors (e.g., FepA, CirA, or Fiu) [47, 48]. Other mechanisms of action of bacteriocins such as the interference in gene expression and protein biosynthesis have been proposed. Examples include interference with DNA (e.g., inhibition of supercoiling mediated by gyrase), RNA (e.g., blocking mRNA synthesis and binding to the 50S ribosomal subunit), and protein synthesis (e.g., modification of amino acids and binding to the elongation

isolated from chicken with *in vitro* antibacterial activities [30, 31].

*enterica*, and *Staphylococcus aureus* using rodent models [39–41].

**8**

factor Tu) [49–53].
