One of the knocks against phage therapy – and I’ve said this myself – is that phage are intrinsically narrow-spectrum agents. Not only do phage typically infect only one species of bacteria, they often don’t even do that. Usually their host range is restricted to a set of strains within a species.
The result of this limited host range is that phage preparations are “cocktailed”. That is, they consist of a mixture of phages that collectively cover the likely range of strains encountered in an infection due to a given bacterial species.
You can see how this raises barriers to adoption. Formulating a cocktail of phages multiplies not only the discovery work required, but all the steps downstream required to turn phage into a product. You need to optimize the composition of the cocktail, then optimize production and purification of each one, then develop adequate QC procedures for each one, and then develop GMP and QC procedures for the final mixture. And the least-stable phage in your mix determines the shelf-life of the entire product.
Once at the clinic, a doctor has to know which bacterial species is responsible for an infection. This is rarely the case for outpatient use, and typically requires 2-3 days of lab workups for inpatients. It’s much easier for doctors just to prescribe antibiotics that will cover the half-dozen bacteria most likely to be the cause of infection.
Even worse, some promoters of phage therapy advocate making custom phage cocktails based on bacteria isolated from individual patients. That is a sure-fire barrier to adoption in all but the most hopeless cases.
Phage host range appears to be mostly a function of cell wall recognition. Phage usually bind to carbohydrate components of bacterial cell walls – techoic acids in Gram-positives, lipopolysaccharides in Gram-negatives. These core carbohydrates have many variants, and phage tend to specialize in binding to one or a few of them, thus limiting their host range.
But there is no law of biology that says that this must be the case. And phage are extremely diverse, with chromosomes chock full of genes of unknown function and unrelated to any other genes in the biosphere. With such diversity and abundance, it’s a safe bet that phage have evolved to fill just about every ecological niche imaginable.
One of those niches would be that of an omnivore – a phage that may not infect any single bacterial species very efficiently, but is “good enough” at infecting many species. Omnivore phages would go a long way toward making phage therapy more practical.
Researchers in Korea appear to have found such a phage. Screening sewage from a chicken farm (a classic phage isolation method), they discovered phage SS3e. This phage lysed 482 of 483 isolates of 22 different serotypes of Salmonella enterica, a notoriously diverse bacterial “species”.
Even more remarkable, this phage lyses pathogenic strains of E.coli and Enterobacter cloacae, and also a few isolates of Serratia marcescens and Citrobacter freundii. It showed no activity versus the few Klebsiella and Proteus strains tested, nor against Gram-negative non-fermenters such as Pseudomonas and Acinetobacter.
The method used to determine susceptibility was fairly crude and strictly qualitative – a drop of phage solution (106/mL) spotted onto a lawn and scored positive if there is clearance. Phage don’t have to be much more than 0.1% efficient to score a positive in this kind of test. An efficiency of plating experiment (in which the number of plaques produced by a fixed number of phage is compared to a reference bacterial host) would be much more informative.
But still.
A recent disappointing phage therapy trial used a cocktail of 12 phages to treat patients with Pseudomonas-infected burn wounds. It failed to show efficacy, and the authors claimed that insufficient host range was the principal problem. I disagree with their conclusion, but limited host range is undeniably a barrier to implementing effective phage therapy.
The particular phage used in this study is probably not a good candidate for phage therapy (Salmonella is an intracellular pathogen and thus not accessible to phage during infections) but could well be used for pathogen control in food processing.
But the important message from this paper is that broad host-range phage are out there. Phage researchers have historically searched for phage using methods likely to yield narrow-spectrum phage. Although these methods are convenient, it is clear that they are not good enough. It’s time to broaden the search to include more of the dark matter of the phage universe.