Microbial pathogens and toxins

Entry to the marine environment

Recorded levels in the marine environment

Fate and behaviour in the marine environment

Effects on the marine environment

Potential effects on interest features of European marine sites

Entry to the marine environment

  • Introduction of infected alien species;
  • Introduction of pathogens directly from ship ballast water;
  • 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 below).
  • 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 species.

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, 1997b).

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 environment

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 pathogens.

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 waters.

Microbial toxins are not routinely monitored in the marine environment.

Fate and behaviour in the marine environment

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 microbial toxins.

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