Exploring Non-Antibiotic Treatment Options for BRD

Background

Antibiotics are effective tools to prevent or treat bovine respiratory disease (BRD) but concerns about antibiotic resistance means that non-antimicrobial options need to be explored. One possible solution may come from phage endolysins—enzymes produced by viruses that specifically attack bacteria (these are known as phages). Phages infect bacteria, hijack the bacterial machinery to manufacture more phages, then produce endolysins (LYS) that cause bacteria to burst. This releases the offspring phages, which can go on to infect other bacteria.

Antimicrobial peptides (AMPs) are tiny protein fragments found in the immune systems of almost all animals. AMPs punch holes in the bacterial cell wall. This causes the bacteria to burst and die rapidly, making it much harder for them to develop resistance. These researchers examined whether the endolysins and AMPs can serve as effective antibiotic alternatives for combatting BRD-associated bacterial pathogens.

Objectives 

• Identify and engineer LYS from phages that target Mannheimia haemolytica, Pasteurella multocida and Histophilus somni, bacterial pathogens associated with BRD.
• Evaluate antibacterial activities of the engineered LYS against multidrug-resistant BRD pathogens including M. haemolytica, P. multocida and H. somni.
• Optimize anti-BRD activities of LYS by fusion with lipopolysaccharide-destabilizing peptides and bovine tracheal antimicrobial peptides.
• Determine the effectiveness of the optimized LYS for controlling BRD in experimentally challenged calves.

What they did

This project investigated two innovative alternatives to traditional antibiotics:

1. Phage Endolysins: enzymes derived from phages (viruses that naturally kill bacteria) which act like “molecular scissors” to cut open bacterial cell walls. These enzymes were engineered to better penetrate the outer membrane of Gram-negative bacteria.
2. Antimicrobial Peptides (WK2 AMP): small proteins that disrupt bacterial membranes.

These novel treatments were tested in the lab for their effectiveness against resistant bacteria and safety when exposed to bovine nasal cells. The WK2 AMP was also tested in mice to see if it could treat active infections. 

what they learned

Engineered phage endolysins showed promise in the lab. Endolysins were successfully identified and produced from phages that target M. haemolytica. The natural endolysins produced by the phages struggled to penetrate the bacteria on their own. However, when fused with polycationic nanopeptides (PCNP)—a special “key” to unlock the bacterial outer membrane—and combined them with EDTA (a helper molecule), they achieved over 99.99% reductions in bacterial counts.

The antimicrobial (WK2) peptide was highly effective in the lab, killing multiple strains of M. haemolytica and P. multocida quickly without harming bovine nasal cells. Pasteurella infections were established in mice. However, a single dose of WK2 was not sufficient to cure the mice or reduce bacterial loads significantly. While potent in the petri dish, WK2 likely requires multiple doses or a different delivery method (e.g., nasal spray or sustained release) to work in living animals.

what it means

There are new antibiotic alternatives on the distant horizon. Engineered phage enzymes can kill drug-resistant BRD bacteria. This opens a new avenue for drug development that does not rely on traditional antibiotics.

Finding a new drug is only step one; optimizing how it is delivered (dosage and frequency) is critical for success in live cattle. Additional research is needed to improve the stability of these treatments inside an animal’s body, develop multi-dose regimens or sustained-release delivery systems to maintain high therapeutic levels in the blood and lungs and develop specific formulations (like nasal sprays) that target the respiratory tract directly.