I.1. BACTERIA: PATHOGENICITY FACTORS – 57
specific lifestyle in a specific niche that happens to be the surface or interior of the host’s
body.
Assessing potential for bacteria-mediated adverse human health effects
In the previous sections it has been argued that deleterious effects that are caused by
pathogenic organisms can be understood as effects of a “lifestyle”, or constellation of
traits, that enables these organisms to colonise and use specific environments in or on the
human or, in general, animal body, as a niche. This line of thinking has been broadened
by Casadevall (2006), who has pointed out that many micro-organisms in the
environment have developed lifestyles that allow them to interact with the other
organisms that they encounter in the environment. The same or similar gene products
may have a role in different lifestyles, e.g. the interaction with fungi, protozoa as well as
vertebrates. Bacterial strains that have no directly apparent role in human pathogenicity,
for instance because they do not survive or replicate at 37°C, may still carry genes that
code for gene products with a potential role as virulence factors in bacteria that are more
compatible with a lifestyle as a human pathogen. DNA exchange between microbial
strains may in the end provide bacteria that thrive in the human environment with new
virulence factors derived from such bacteria. This complicates the risk/safety assessment
of deliberate release of environmental strains, particularly if these strains have been
subject to genetic engineering. The following is intended to help the assessor negotiate
these complications.
Risk/safety assessment of environmental release of bacterial strains
to determine whether these may cause adverse human health effects
Environmental release of bacteria should be preceded by a risk/safety assessment.
Risk assessment usually comprises four steps: 1) hazard identification; 2) hazard
characterisation (e.g. dose-response assessment); 3) exposure assessment (dose,
concentration, survival); and 4) risk characterisation. In the risk assessment of
environmental releases of bacterial strains, one aspect that has to be taken into account in
hazard identification and hazard characterisation is the pathogenic potential of the
bacteria to cause adverse human health effects.^4 The WHO Laboratory Biosafety Manual
(WHO, 2004, Chapters 2 and 16) provides helpful considerations on the risk/safety
assessment of (potentially) pathogenic organisms. These considerations apply primarily
to laboratory settings, but they can be easily adapted and applied to environmental
settings.
For genetically engineered bacteria, the risk group of the species is a first
approximation of the degree of bacterial pathogenicity in humans. But assessing the
degree of pathogenicity of a bacterial strain calls for an unequivocal identification of the
location of the strain in the spectrum from clear non-pathogen to clear pathogen.
This should be done with caution. Truly non-pathogenic bacteria will lack the ability to
survive in a human host (with the exception of commensal bacteria), or cause any adverse
effects. Bacteria that are incompatible with the human environment e.g. bacteria that
cannot survive at temperatures between 30-42°C, or that are exclusively phototrophic or
lithotrophic would be expected to be non-pathogenic. Still, one should be careful drawing
this conclusion. For instance, lithotrophic bacteria have been found in infections
associated with surgical implants in the human body (Dempsey et al., 2007). Indeed,
Casadevall (2006) has pointed out that bacterial strains that have lifestyles that do not link
them to pathogenicity in humans can carry genes that code for gene products with a
