Dr. James Evermann
Lecture 7: Antibiotics Benefits and Dangers
Antibiotics
The way it is determined if a bacteria is sensitive to a particular antibiotic is called antibiotic sensitivity or susceptibility testing. A colony of bacteria from the agar plate is spread on another agar plate to for a "lawn" of bacteria. Then you place antibiotic discs on the lawn of bacteria. An antibiotic disc contains antibiotics on a thin piece of paper. The disc can be coated with any antibiotic. You can determine if the bacteria are resistant or susceptible to the antibiotic on the disk. MIC (minimal inhibitory concentration) or sensitivity, which is what they are talking about. The "cleared area" is where bacterial growth has been attenuated and you know the concentration of antibiotic is at or above the minimum inhibitory concentration in the clear zone. This zone is called the zone of inhibition and can be measured. Depending on the strain of bacteria this zone maybe small or large.
Around the zone of inhibition is another zone in which the bacteria are not inhibited to any great extent. This is where the resistant bacteria start. There is no growth in the zone of inhibition around that individual colonies of bacteria start to manifest themselves. These are the resistant bacteria. This happens in the agar gel plate and it happens in the animal’s body. It is resistance.
An antibiotic is an antimicrobial substance produced by either a living microorganism or synthetically produced. In 1929, Fleming observed that a fungus, Penicillium notatum, when present on a petri plate, strongly inhibited the growth of Staphylococci species. This substance was referred to as penicillin. Antibiotic resistance occurs when bacteria that were sensitive to an antibiotic become resistance. Although there are obvious benefits to antibiotics are also dangers of antibiotics.
Mechanism of Action
Antimicrobial drugs are divided into two categories based upon their effects on bacteria.*
Antibiotics may have a common or generic name as well s trade name that's specific to the manufacturer. Tetracycline. is a broad spectrum antibiotic. It attacks both the gram positive and the gram-negative bacteria. The problem is that they are contraindicated with certain other antibiotics. You don’t give tetracycline with penicillin because the combination of bactericidal and bacteriostatic antibiotics are contraindicated. Tetracycline must be discontinued 28 days prior to slaughter. This is the withdrawal time. Tetracycline is not to be used in lactating animals.
Many antibiotics can only be obtained by having a veterinarian write a prescription. It is up to you to be judicious about the use of antibiotics. An overuse of antibiotics is going to generate some bad bugs, that are resistant to antibiotics.
Penicillin has a very selective effect upon the gram-positive bacteria. It is still widely used. Streptomycin is now streptomycin sulfate. Strep used to be combined with penicillin as pen-strep. Widespread use of pen-strep led to resistance in bacterial populations. If you go home and have a bottle of streptomycin-sulfate on the shelf, you might want to get rid of it.
Antibiotic Resistance:
An ever-present danger
The emergence of bacteria resistant to antimicrobial treatment is of great concern for two major reasons. The first is when resistance develops to
a previously used antibiotic, then a suitable alternative antibiotic must be used. Second,
the presence of resistance is of potential public health concern.
Bacteria have two types of genetic structures, that may confer resistance: chromosomes and plasmids. Chromosomal resistance to antibacterial substances depends upon mutation in the bacterial genes. The antibacterial substance(s) act as selective agents that allow the resistant mutants to emerge. Plasmid-mediated resistance (R-factor or acquired resistance) is more complex. Plasmids may contain 20-500 genes that can carry resistance to a number of different antibacterial substances, as well as specific virulence factors.
The definition of an antimicrobial drug may be extended to include
antivirals (viruses);
antimycotic (fungi) and antiprotozoals (protozoa). If you see the phrase
"Third generation" associated with an antibiotic it indicates that
the antibiotic is
a synthetic molecule. Penicillin is a
natural product produced from mold. The newer generations of antibiotics are
usually synthetic, meaning that have been chemically synthesized in the laboratory.
Fungi are no longer used to produce antibiotics.
Recall that bacteria are prokaryotic which means they do not have a true nucleus. They have a cell wall. They have ribosomes. Their nucleic acid is just all bundled up inside. Resistance can occur by a mutation in the chromosome, by insertion. This usually only infers resistance to one antibiotic. Plasmid-mediated resistance is more serious as it is confers resistance to many antibiotics. These are also referred to as R-factors or resistance factors. Plasmids are extra-chromosomal. That means they are outside the chromosomes. Bacteria may have multiple plasmids. Plasmids may contain up to 20 to 500 genes that can carry resistance to a number of different antibiotics. These plasmids are bad news. If a bacteria has a number of different plasmids associated with it the bacteria maybe resistant to just about every antibiotic known to humans. That is the danger of Salmonella typhimurium DT-104.
There is a difference between chromosomal mutation that can produce resistance to one antibiotic versus plasmid-mediated resistance that confers protection against multiple antibiotics. Veterinarians, animal scientists, and physicians have to be careful about the indiscriminate use of antibiotics because it selects for resistant bacteria. In other words, if we are not responsible in the administration of antibiotics, we kill all the susceptible bacteria and allow the resistant bacteria to grow with out any competition. It is important for you to recognize that there are three mechanisms of resistant bacteria: chromosome mutation, plasmid mediated, and antibiotic overuse selection.
Relevance on the Farm
The relevance of plasmid-mediated resistance in animal agriculture concerns the
following: 1) intestinal infections, in which the reservoir of the R-factors may be
carried by saprophytic bacteria in the gut (E. coli); 2) the use of low levels of
antibiotics (as in animal feeds) or improper dosing regimens, which may lead to high
occurrence of R-factors in a bacterial population (Salmonella spp.); and 3) the
indiscriminate use of antibiotics, which may eliminate the effectiveness of antimicrobial
substances in the future.
Steps to minimize development of resistance – there are 6 ways to minimize or avoid development of antimicrobial resistance. This isn’t going to prevent resistance; it is just going to minimize it.
Summary
Antibiotics have been one of the major advances in medical and veterinary care in the past
50 years. Unfortunately, the rapid rate at which bacteria adapt to the antimicrobial
substances, resulting in resistance, calls for a unified judicious use of antibiotics now
and in the future. It is highly advisable to always consult with a veterinarian when using
antibiotics in animals and to follow the regulations on package inserts for determining
what the withdrawal times are for various antimicrobial drugs prior to slaughter. These
times may range from 24 hours to 30 days depending upon the ½ life of the drugs in body
tissues.
Question: How long do you grow the bacterial plates?
Dr. Evermann’s response: Usually 24 hours. There has to be enough time for this lawn
to develop. The lawn develops in 24 hours. It takes that long for bacteria to replicate.
Then you look at resistance or susceptibility of the antibiotic by measuring the zone
of inhibition.
Question: Are sensitivity tests available on pamphlets that come with the
antibiotics or do you send for them?
Dr. Everman's response: Generally speaking, the inserts should have that information on
then; what they are resistant to (unable to kill) and what they are susceptible to (able
to kill).
Question: Do the information packets that come with antibiotics tell you
about their effectiveness against certain diseases such as Johne's?
Dr. Evermann: These tests measure the resistance and susceptibility of a given bacteria
to antibiotics. Johne’s testing does not measure
sensitivity to antibiotics. Mycobacterium tuberculosis is not
treated with antibiotics. The testing strategy for Johne’s is to measure
antibodies to the bacterium and to determine if the animal is infected or not.
References
Merck Vet Manual, 8th edition, 1998, pp 1738-1745.
Hoopes and Thwaits, 1997, pp 213-221.
Levy SB, The challenge of antibiotic resistance. Sci Amer March 1998, p 46-53.
Witte W, Medical consequences of antibiotic use in agriculture. Science 279:996-997, 1998.