Salmonella control on farm – Know your enemy

This is part 2 of an article based on a presentation prepared for an Schering-Plough Animal Health roadshow about salmonella, sources of contamination, laboratory testing, the diseases and control.

Knowing your enemy

  • Salmonellae are bacteria that can cause infectious disease
  • Other causes of infectious disease include (smallest to largest):
    • Viruses
    • Chlamydia
    • Coccidia
    • Bacteria
    • Worms
    • Mycoplasma
    • Mites
    • Fungi

Bacteria are very small and can live on or in cells of the body. They are characterised by a solid outer wall to which are attached flagellae. These enable the bacterium to move and smaller ones stick the bacterium to cell surface. They can be seen using a light microscope. Using special stains can make this easier. Salmonellae can be cultured in the laboratory.

Chlamydia and mycoplasmas are specialised types of bacteria. Bacteria are the smallest living infectious organisms.

Viruses are non-living. These need the living cells to replicate, bacteria, the chicken. They effectively hijack the cell so that rather than make normal cell components, viral components are made instead. Often this can result in cell death.

A black and white electron micrograph of bacteriphages that infect salmonellae.
An electron micrograph of bacteriphages that infect salmonellae.

Viruses can infect bacteria. The black and white picture is an electron micrograph of bacteriophages that infect salmonllae. The picture below shows salmonellae growing on the right.

Salmonella infected with a virus. preventing growth.
Cultures infected with bacteriophages fail to grow. Left is infected picture is infected.

If the cultures are infected with a bacteriophage the salmonellae die, fail to grow so there are less to be seen. These viral bacteriophages are used to further identify the salmonella serovars. This helps track infections and bacterial changes.

Can you identify your enemy

Scientist tell us that salmonellae belong to:

The family EnterobacteriacaeaIntestinal bacteria – E.coli, Proteus sp.
Gram NegativeStain red
Aerobic or facultatively anaerobicGrow best with oxygen
AsporogenousDo not produce spores
Rod shapedNot spherical
Grow on artificial mediaCan be cultured

So we can identify their presence and separate them from other gut bacteria.

Heat5 – 45c. Roasting, boiling but soft yolks a problem.
pH4.0 – 9.0 optimum 7.0
LitterSurvive 7 – 16 months at 20 – 25c
FeedSurvive 18 months
ChemicalsMost effective is formaldehyde, others help

S.Senftenberg are more heat resistant than most. Internal temperature of 79C kills St in roasting chickens. 60C for 5 minutes killed 100 million bacteria per gram of ground chicken meat. Prior refrigeration increases susceptibility of Se to temperature.

Ultrasound, irradiation and electrical stimulation also help.

Salmonellae lab test classifications

Into 2 species:

  • S.enterica
    • This is futher divided into 6 sub species e.g
      • Host adapted, non-motile – S.Pullorum, S.Gallinarum
      • Non Host adapted, motile 2500+ serovars which includes
      • S.enterica subsp. enterica serovar Typhimurium
  • More commonly known as Se or St

The import species for us is S.enterica. This species is further subdivided into 6 subspecies. The important one for us being enterica. Thus officially St would be known as S.enterica subsp enterica serovar Typhimurim / Enteritidis / Virchow as appropriate. These are still usually shortened to Se and St.

The importance of being able to differentiate salmonellae using specific antibodies enables us to study how the infection spreads and develops. For instance Se belongs to serological group D and St to Group B.

Salmonella identification

  • Salmonella enterica subsp enterica serovar Pullorum (Sp)
    • Disease of chicks eradicated from commercial flocks.
  • S.Gallinarum (Sg)
    • Disease of laying birds eradicated from commercial flocks.
  • S.Enteritidis (Se)
    • Opportunistic pathogen of fowl, commonest cause of salmonella food poisoning in humans.

This group also includes Pollorum Disease, Fowl Typhoid, S.Dublin (a cattle salmonella) and many others. Using these antibodies enables us to distinguish between the subtle differences on the surface of these bacteria.

In the lab we can culture salmonellae and identify which of the 2500 we are dealing with.

About David Parsons 19 Articles
David Parsons began his veterinary career in mixed practice which triggered his 39-year passion for poultry. Following positions as a veterinary research officer in the Poultry Department at the government’s Central Veterinary Laboratory and then as a poultry company veterinarian, he set up his own poultry veterinary practice in the southwest of England in 1985. He obtained his MSc in Applied Immunology in 1981, Certificate in Poultry Medicine and Production in 1989 and a Nuffield Farming Scholarship Trust to study the“Status of diseases specific to poultry and their control in Europe” in 1991.

He has been an Honorary External Lecturer at the University of Bristol Veterinary School on poultry medicine and production since 1999,a lecturer on the Institute of Animal Health’s poultry disease course since 2000 and is a regular monthly contributor of veterinary articles for backyard poultry keepers in the Practical Poultry magazine.