Antibiotic Resistance

The questions:

The Answers:

Q What are antibiotics?

A Antibiotics are chemicals that inactivate or damage bacteria.

Most are naturally produced by bacteria and fungi, others are man-made but have the same effect. The term antibiotic is popularly used to describe this whole range of chemicals. Strictly, it should only apply to the naturally derived chemical substances: we will use this popular terminology here.

Antibiotics are administered to animals to kill bacteria that are causing infections (therapeutic and prophylactic antibiotics).

Antibiotics are not effective against viruses.

Q What is resistance?

Resistance is the ability of an organism to survive in the presence of concentrations of a chemical which are normally lethal to organisms of that species. When bacteria are exposed to an antibiotic various things can happen:

1) They can be killed, or;

2) They can be weakened, or disabled, thus making them easier for the animal's own natural defences to kill, or;

3) They can remain unaffected, or resistant.

What actually happens depends largely on whether the antibiotic is of the correct type to work against that bacteria and whether the antibiotic is present in a sufficiently high concentration. Not all antibiotics will work against all bacteria and some bacteria require higher concentrations of antibiotic to kill them: resistance is a natural phenomenon and was present in bacteria before antibiotics were used.

The amount of antibiotic provided can be increased to meet the concentration required to kill the bacteria. Bacteria that require higher concentrations of antibiotic to kill them are less sensitive but are not necessarily resistant. Antibiotic resistance is commonly used to describe the situation when the concentrations of antibiotic needed to kill the bacteria cannot be achieved at the site of the infection.

It is important to remember that if another antibiotic type is used to which the bacteria are sensitive then they will be killed. Just because a bacteria is resistant to one antibiotic does not mean that it is resistant to all antibiotics. However, some bacteria can be resistant to more than one antibiotic and these are called multi-resistant. It is these bacteria that cause the most concern.

Q How does antibiotic resistance come about?

A This is a natural phenomenon and has been around as long as bacteria. Recently, bacteria isolated from a glacier formed long before the discovery and use of antibiotics have been found to be resistant to some modern antibiotics. A certain level of inherent bacterial resistance to antibiotics must therefore be expected but surviving bacteria will be those that are of low sensitivity or are resistant.

It is difficult to predict how quickly resistance will develop as it depends amongst other things on the type of antibiotic, the type of bacteria, the level of exposure those bacteria have to the antibiotic and the ability of the resistant bacteria to survive and replicate.

After resistance has developed in a particular bacteria it may in time disappear or the resistant strains may be replaced by susceptible bacteria. There is still much that is unknown about what happens to antibiotic resistance after it has developed. It is the subject of much ongoing research.

Q What are the effects of antibiotic resistance?

A When resistant bacteria cause infections, the choice of antibiotic that can be used to treat that infection is reduced. If immediately recognised, the doctor or vet can use an alternative antibiotic with little or no risk to the patient. If not recognised until the initial course of antibiotics fails to work, the delay between initial diagnosis and the start of successful treatment can result in unnecessary suffering and a deterioration of the patient's condition.

Q Are resistant organisms always a problem?

A A resistant bacteria is one that carries a defined resistance mechanism, such a bacteria will be termed microbiologically resistant. In many cases microbiologically resistant bacteria will still be susceptible to antibiotics because the concentration of antibiotic at the site of infection following therapy will be sufficient to kill the bacteria. These bacteria are microbiologically resistant but clinically susceptible. Microbiological resistance does not necessarily equate to clinical resistance.

Q Where is antibiotic resistance a problem?

A Antibiotic resistance, whilst a serious issue in some intensive care hospital settings and with chronic infections in humans such as MRSA and TB, is rarely an issue in animals. In the community, concern exists about food-borne pathogens. Fortunately, there is only a limited number of these that we need concentrate upon, such as Salmonellae, Campylobacter and Enterococci. The former two are rarely treated with antibiotics in man.

Antibiotic resistance could be indirectly transmitted via organisms normally present in the healthy gut, such as E coli and Enterococci. This is a problem of hygiene rather than antibiotic use and can be prevented by good hygiene in the kitchen and where animals and food are handled.

Antibiotic resistance is a world-wide problem but is worse in some parts of the world than others. Particular resistant bacteria can be a major problem in some countries but not in others; for example, vancomycin resistant enterococci (VRE) are a major problem in the USA but are not so in much of western Europe. This can be explained by the intensive use of vancomycin in the USA over the past 15 years. Vancomycin is not used in animal medicine.

However, widespread and extensive use of a particular antibiotic does not always lead to resistance. Cloxacillin, a member of the penicillin family of antibiotics, has been used by vets and farmers for almost 30 years to treat and prevent mammary infections in dairy cows at the end of lactation (dry cow therapy) yet resistance to cloxacillin is almost non-existent – even in bacteria such as Staph. aureus that are well recognised as being able to rapidly develop resistance to antibiotics.

Q Is antibiotic resistance a growing problem?

A Yes it is, but bacteria have always been able to develop ways to protect themselves from antibiotics so it is inevitable that as long as antibiotics continue to be used resistance will continue to develop.

New antibiotics are being developed. Many new therapeutic options are currently in development world wide to relieve the over-reliance on existing products for man. However, it must be remembered that many of the oldest and most widely used antibiotics are still being effectively used.

Clinically, antibiotic resistance is really only a growing problem in man, much of it being in hospitals where patients are critically ill or are immuno-suppressed. Current research suggests that the bacteria causing these infections in hospitals are acquired from within the hospital environment and are rarely brought in from the community or from contact with animals or food.

Several infections in animals resemble those found in man, so it is no surprise that some antibiotics used in man are also helpful in treating animal infections. However, many compounds used to treat animals are quite different from those used in man, and even where there is a chemical similarity, there is a lack of evidence that human use of these antibiotics is in any way affected by their use in animals.

Q What effect would banning use of specific antibiotics in animals have on antibiotic resistance in man?

A The short answer is little or none.

Extensive research continues to be performed both here in the UK and throughout the world with the aim of providing us with the answer to this important question.

All the available evidence suggests that the use of antibiotics in animals has had little or no impact on the incidence of antibiotic resistance in human infections with bacteria such as enterococci, known to be carried by animals.

Indeed, recent studies show that resistance amongst human enterococci bacteria to an antibiotic used for over 25 years in poultry and pigs throughout the world is still at the very low levels we would expect had the animal antibiotic never been used. This is not surprising since enterococcal strains from animals do not survive in the intestine of man.

It has long been known that some types of bacteria, e.g. Salmonella and Campylobacter, can pass from animals to man. It must be remembered, however, that the majority of food poisoning outbreaks caused by these bacteria are not treated with antibiotics.

And although the majority of food poisoning outbreaks caused by these bacteria are not treated with antibiotics, any increased use of antibiotics in humans due to increased contamination has important implications for risk/benefit assessment. As healthy food comes from healthy animals, if potentially unhealthy food were to enter the food chain (for example if infection went untreated) reduction of antibiotic use in animals could lead to more use in humans and hence be counter-productive.

It is important that any legislation is based on sound scientific assessment, taking into account the whole picture, rather than considering any action in isolation.

Q Does long term antibiotic use always produce antibiotic resistance?

A No. As the example of cloxacillin use for dry cow therapy shows us, resistance to an antibiotic cannot be predicted just by looking at the duration or frequency of antibiotic use. Many other factors need to be taken into consideration and it is precisely because scientists do not fully understand how and why resistance develops that so much research is ongoing both in animals and man.

Nevertheless, the medical and veterinary professions acknowledge the need to use antibiotics responsibly. There are several codes of practice for responsible use of antibiotics in all animals, from worldwide guidelines to those developed in the UK. The industry has had a major role in the development of the UK guidelines produced by RUMA (the Responsible Use of Medicines in Agriculture Alliance).

Updated October 2005