Like little chemical factories
Bacteria are minute organisms invisible to the naked eye. There are
many different types of bacteria and they are probably the most diverse
group of organisms on the planet each like a little chemical
factory. Very few species are pathogenic (disease-causing). Those that
are harmful can be divided into two basic types; Gram-negative and
Gram-positive. This simple grouping is based on a staining technique in
which Gram-negative bacteria stain red, while the Gram-positives stain
blue because of differences in the cell-wall structure.
using Gram differential staining method
Stained Gram-positive bacteria viewed at high
Stained Gram-negative bacteria viewed at high
Most bacterial fish pathogens, such as Aeromonas, Pseudomonas,
Vibrio, Flavobacterium and Cytophaga are Gram-negative
bacteria. These are the bacteria that are usually involved with
bacterial disease such as ulcers, fin rot, acute septicaemia and
bacterial gill disease. Less common pathogens are Mycobacterium and
Norcardia sp. which cause chronic granulomas (or abscesses).
How do bacteria cause disease?
Although they are incredible small most pathogenic bacteria have
tremendous reproductive potential. They simple divide in two and once
each half has re-grown sufficiently they divide again. Under ideal
conditions the reproductive cycle can be as little as 20 - 30 minutes,
which means that just one bacterium can multiple to several million
within 24-hours or so!
As I have said, bacteria are like little chemical factories. Some
bacteria produce toxins that are excreted into the blood and tissues of
the host. Other bacteria, particularly the Gram-negatives do not secrete
a soluble toxin but make an endotoxin that is liberated when the cell
dies and disintegrates. These endotoxins are usually lipopolysaccharide
structural components of the bacterial cell-wall (specifically the lipid
A portion). In addition to toxins the virulence of many bacteria is
partly due to the production of extracellular enzymes, which attack
healthy fish cells.
So although tiny, the net effect of millions of bacteria can quickly
overwhelm the defences of the fish host which is why early treatment
is so vital if the fish is to survive.
How do antibiotics work?
Antibiotics are chemical substances produced by microorganisms that
either destroy (bactericidal) or
inhibit the growth of other microorganisms (bacteriostatic).
Antibiotics can be either broad spectrum,
which means that they are active against a wide range of
microorganisms both good and bad. Narrow
spectrum drugs target a specific group of microorganisms and
are able to interfere with a metabolic process specific to those
organisms. In general antibiotics work by;
Preventing the synthesis of bacterial
cell-wall components typically penicillin
Damaging the bacterial cytoplasmic
Interfering with protein or nucleic
acid synthesis typically tetracyclines
However, the microorganisms under attack dont take all this
sitting down. Many are able to produce enzymes that neutralise
antimicrobial compounds. Others can acquire resistance to weak,
short-term or repeated exposure to antibiotics. For example there are
now high levels of resistance to oxolinic acid and oxytetracycline
because of overuse and misuse.
What antibiotics are available?
Every country has its own regulations on the control of antibiotics
and therefore availability will vary. The range of drugs available or
licensed for use with fish is generally small. In the UK all antibiotics
are classed as POMs (Prescription only Medicines) and are only available
under veterinary supervision. Among the antibiotics in common use are;
that include amoxycillin and ampicillin. These are bactericidal in
action and are effective against Gram-positive and Gram-negative
bacteria, but not very effective against Pseudomonas.
Chloramphenicol is a broad
spectrum antibiotic, but again it is not usually effective against
Pseudomonas. Because it is used in human medicine it is
illegal for use in food animals in most countries. Some people are
sensitive to the drug so rubber gloves should be worn when it is
There are several potentiated
sulphonamide drugs in which a sulphurdrug is combined
with trimethoprim. They are broad spectrum and mainly
bacteriostatic. They are well absorbed from the gut when used with
medicated food but poorly absorbed from water. They can sometimes
form crystals in urine and should not be used where there is
already kidney damage. They should not be used at the same time as
organophosphates. Co-trimazine is a combination of trimethoprim
and sulphadiazine and Borgal is a combination of trimethoprim and
sulfadoxine. They are effective against Aeromonas and other
Gram-negatives, but not particularly effective against Pseudomonas.
Gentamicin is a broad
spectrum antibiotic with a bactericidal effect. It is useful
against all Gram-negatives including Pseudomonas but can be
toxic to kidneys. There is concern that bacteria may acquire rapid
resistance, so it would not be the drug of first choice but does
have a use when other drugs do not work.
Nitrofurans are a group of
synthetic antimicrobials. Both nifurpirinol and nitrofurazone are
effective against many fish pathogens. They are well absorbed
through the skin, making them ideal for bath treatments. However,
they are carcinogenic and mutagenic and their use with food fish
is illegal in most countries.
bacteriostatic drug with a broad spectrum of activity. However,
there is widespread resistance and it has little effect on Pseudomonas.
The long-acting injectable formulation has caused sterile
cysts at the injection site. When used for bath treatments it
readily chelates calcium and magnesium ions found in hard water,
significantly reducing its effectiveness. It is also
light-sensitive when used as a bath treatment, turning brown as it
decomposes. The degraded form can be toxic to both fish and
humans. Some studies show that oxytetracycline is
immunosuppressive in some fish species.
The quinolones are narrow
spectrum range of drugs acting mainly against Gram-negatives such
as Pseudomonas and Aeromonas. The first generation
quinolones include oxolinic acid to which there is now widespread
resistance. Newer quinolones include enrofloxacin
and sarafloxacin. These are
effective against Pseudomonas and Aeromonas and as
yet there is little resistance. All quinolones are chelated in
hard water, so they are not practical for bath treatments. They
are well absorbed through the gut, making them useful for
Bacterial resistance is an issue that needs to be considered when
choosing an appropriate antibiotic. While it may be necessary to start a
course of treatment based on personal experience, it is also prudent to
have bacterial identification and antibiotic sensitivity tests carried
out at the same time. At worse this simply confirms the treatment
choice, while possible saving valuable time and money if the initial
course of action is ineffective.
A sterile swab is used to take a bacterial sample from either the
posterior kidney or other organ of a newly dead fish, or alternatively
from a body ulcer. When taking a swab of a body ulcer it should be
sampled from the infected rim of the ulcer only.
The swab is then sent to a specialist laboratory for analysis and
testing. Your local veterinarian can arrange this. The results from a
body ulcer can be difficult to interpret because of the problem of
secondary pathogens. It is quite common for samples from body lesions to
show a mixed growth of several bacterial species and takes some
experience to determine the relevance of the results.