Therapeutic Bacteriophages for Gram-Negative Bacterial Infections in Animals and Humans

Abstract

Drug-resistant Gram-negative bacterial pathogens are an increasingly serious health threat causing worldwide nosocomial infections with high morbidity and mortality. Of these, the most prevalent and severe are Pseudomonas aeruginosa, Klebsiella pneumoniae, Escherichia coli, Acinetobacter baumannii, and Salmonella typhimurium. The extended use of antibiotics has led to the emergence of multidrug resistance in these bacteria. Drug-inactivating enzymes produced by these bacteria, as well as other resistance mechanisms, render drugs ineffective and make treatment of such infections more difficult and complicated. This makes the development of novel antimicrobial agents an urgent necessity. Bacteriophages, which are bacteria-killing viruses first discovered in 1915, have been used as therapeutic antimicrobials in the past, but their use was abandoned due to the widespread availability of antibiotics in the 20th century. The emergence, however, of drug-resistant pathogens has re-affirmed the need for bacteriophages as therapeutic strategies. This review describes the use of bacteriophages as novel agents to combat this rapidly emerging public health crisis by comprehensively enumerating and discussing the innovative use of bacteriophages in both animal models and in patients infected by Gram-negative bacteria.

Keywords: Gram-negative bacteria; antibacterial resistance; bacteriophage; multi-drug resistance; phage therapy.

Figure 1A.

Mechanisms of bacterial resistance to antibiotics.

Figure 1B.

Life cycles of phages. Bacteriophages can either be lytic, in which case they cause lysis of the bacterial cell and release progeny within a short period of time, or lysogenic, in which case they inject their genome into the bacterial genome and stay dormant for a while until they get activated and release progeny. For therapeutic purposes, the preferred type of phage is lytic, as killing of the bacteria within a short period of time is necessary in treatment of bacterial infections.

Figure 2A.

Conceptual model of the effect of bacterial infection on phage (Φ) kinetics in blood and tissues. When phage encounter their bacterial hosts (bh) in infected animals, they replicate, and therefore require a longer time to be cleared from the system, in comparison to that of uninfected animals, where there are no bacteria that phage can infect. This self-replication property extends their therapeutic effect, as it leads to higher titers in tissues and blood, potentially leading to a reduced need for many doses, when compared with conventional antibiotics.

Figure 2B.

Conceptual model of the kinetics of conventional antibiotics in blood and tissues. In comparison to bacteriophages, conventional antibiotics cannot replicate and are cleared from the system within the same time, regardless of whether or not a bacterial infection is present in animals.

Figure 3.

Possible mechanisms of enhanced antimicrobial activity of liposome-entrapped bacteriophages (LEBs). The chemical composition and structure of liposomes of LEBs may be engineered for polarity, size, and lipophilicity to evade macrophage uptake by liver and spleen to prolong the plasma half-life of bacteriophages, to enhance intracellular uptake of LEBs by bacteria-infected cells, and to increase affinity to bacterial outer cell membranes. Moreover, LEBs reach the disrupted capillary bed of infected burn wounds where they permeate the tissues and release bacteriophages.