Microbial pathogens and toxins
Entry to the marine environment
- Introduction of infected alien species;
- Introduction of pathogens directly from ship
- Effluent disposal;
- Translocation/release of infected native species
from other sites.
Modes of action:
- Direct infection by pathogenic bacteria present
in the water or river sediments;
- Infection of stressed or debilitated animals
by opportunistic pathogens in the water or sediments;
- Release of endo- or exotoxins by bacteria (see
- Bacterial toxins
Most bacterial toxins can be divided into two basic
types (Todar 1997a,b):
- Proteins (exotoxins), which may be released
into the environment by pathogenic bacteria.
- Lipopolysaccharides (endotoxins), which form
part of the cell walls of Gram-negative bacteria.
Most of the protein toxins are thought of as exotoxins,
since they are released from bacteria and act on
host cells at a distance (Topley and Wilson 1990).
Typically, they are released during exponential
growth, and are specific to a particular bacterial
Bacterial protein toxins are often very potent,
retaining high activity at very high dilutions.
The site of damage caused by the toxin usually indicates
the location of the substrate for that toxin, but
while some protein toxins have very specific cytotoxic
activity, others (as produced by staphylococci,
streptococci, clostridia, etc.) exhibit broader
activity, being able to cause non-specific death
of many cell types and tissues.
The group of filamentous bacteria, known as actinomycetes,
contains a number of known exotoxin-producing genera,
such as Corynebacterium, which do not need
to be dominant members of the microbial community
to have a toxic effect.
Although the term endotoxin is occasionally used
to refer to any cell-associated bacterial toxin,
it should be (and is usually) reserved for the lipopolysaccharide
(LPS) complex associated with the outer envelope
of Gram-negative bacteria, such as E. coli, Salmonella,
Shigella and Pseudomonas (Todar, 1997b,
Topley and Wilson, 1990). LPSs can elicit
a variety of inflammatory responses in animals (Todar,
Gram-negative bacteria probably release minute
amounts of endotoxin during growth. For example,
it is known that small amounts of endotoxin may
be released in a soluble form, especially by young
cultures. However, for the most part, endotoxins
remain associated with the cell wall until disintegration
of the bacteria.
Recorded levels in the marine
Microbial pathogens of humans are monitored in
the marine environment by measuring indicators of
human faecal contamination (e.g. faecal coliforms)
in water and in biota (mainly shellfish) destined
for human consumption. The presence of these indicator
organisms is used to indicate the presence of other
human derived microbial pathogens. The principal
aim of the monitoring and the control mechanisms
is to prevent humans coming into contact with these
The principal monitoring of the marine environment
for micro-organisms is associated with assessing
compliance with standards laid down in the Bathing
Waters Directive and the Shellfish Waters Directive.
This monitoring is limited to designated bathing
waters in the bathing season and to designated shellfish
Microbial toxins are not routinely monitored in
the marine environment.
Fate and behaviour in the marine
The marine environment is hostile to most microbial
pathogens and they will rapidly die off, especially
in the presence of sunlight. They do become associated
with suspended particles and can accumulate to some
extent in sediments and survive for days or weeks.
Microbial pathogens can accumulate in filter feeding
organisms to levels that can be harmful to humans
and perhaps other consumers (e.g. birds).
Effects in the marine environment
In theory, almost any plant or animal is at risk
from microbial pathogens, albeit that most attention
has focused on the causes of fish kills and illnesses
of marine mammals, although monitoring of shellfish
populations means that more information probably
exists about this group of organisms than any other
collective group of marine fauna.
In marine waters, species of bacteria from the
Aeromonas, Alteromonas/Pseudomonas and Vibrio
groups have been detected in elevated numbers
during fish kills associated with Ptychodiscus
brevis red tides (Buck and Pierce
1989). Microbial pathogens are also regarded
as a potential threat to bottlenose dolphins by
Grellier et al (1995), while Thompson et
al (1997) suggested that pathogens associated
with the discharge of raw sewage from the town of
Inverness could have been responsible for pathogenic
infection of harbour seals.
Potential effects on interest
features of European marine sites
Potential effects include:
- potential for pathogenic infection of sea birds
and Annex II sea mammals, resulting from exposure
to microbial pathogens in food organisms or in
the water column;
- potential for sub-lethal and lethal effects
in marine organisms, resulting from exposure to