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:
  1. Insensitive due to inherent or acquired resistance.
  2. Phage carrier (lysogenic). In this state the bacteria are resistant to another phage infection
  3. 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