Bacteriophage (bacterial viruses)
Bacteriophage are the stuff of a cheese maker's nightmare. Like all viruses, bacteriophage (hence forth abbreviated to phage) are parasites, that is, part of their life cycle is dependent on the host bacteria. Here's a few facts about their characteristics and how they can be controlled.
- Extracellular phage, that is, phage particles existing separate from their bacterial hosts are called mature or resting particles.
- Resting particles are sperm shaped, < 1 micron in length.
- Resting particles consist entirely of DNA (genetic material) and protein. The basic construction is a DNA core enclosed in a protein sheath.
- The basic life cycle, called the lytic cycle, is:
- The phage attaches itself to the bacterial cell wall by its tail, bores a hole in the wall with the help of enzymes and injects its DNA into the cell. The protein sheath remains outside the cell.
- From the moment of invasion the bacteria begins to reproduce phage DNA and protein in addition to its own.
- Nucleic acid and protein strands assemble themselves into new phage particles which eventually lyse the cell (break it open) to release the phage particles into the medium. A new generation of resting phage are now available to repeat the lytic cycle
- Sometimes infection occurs without lysis resulting in a lysogenic culture where infected cells survive and reproduce infected daughter cells. Therefore, cheese cultures can exist in one of three states with respect to phage sensitivity:
- Insensitive due to inherent or acquired resistance.
- Phage carrier (lysogenic). In this state the bacteria are resistant to another phage infection
- Phage sensitive in which case the phage will grow quickly and may terminate the culture. Culture growth will stop when phage levels reach 103 to 107 per ml.
- Phage have a short latent period (reproduce as quickly as every 30 to 50 min) and a large burst size (each lysed cell will release 50 to 100 new phage).
- Phage are quite strain specific which is the reason for culture rotation. As many as 10 different cultures may be rotated on a daily basis.
- Culture failure due to phage can be recognized by normal acid development initially followed by a decrease or termination of culture growth at a later stage. This is different than inhibition due to antibiotics which can be recognized by no or slow initial growth; if inhibition is not severe, culture growth and acid development by resistant strains or mutants may increase with time.
Summary of phage control measures
- Use aseptic techniques with proper culture room.
- Rotate cultures daily and/or use defined phage resistant strains.
- Use phage resistant media for culture preparation.
- Use direct-to-vat culture to avoid contamination during transfers.
- Use a mixed strain culture of two closely related strains.
- Remove and dispose of whey daily
- Routinely check for presence of phage using a culture activity test with the culture currently in use and some whey from the most recent vat