Microbiological agents are universally
distributed in nature and make up the predominant part of life on earth. They
occur in soil and water as well as in plants and animals, but also in the most
hostile habitats like hot springs and deep ocean vents. The great majority of
microbial agents are harmless and many are beneficial. Some of their properties
have been used for fermentation in food processes for centuries, while others
are applied in biotechnological processes. However, a few hundred of these
agents are pathogenic or toxic, which means they are capable of causing disease
in man, plants, or animals. Some of them can to be considered as biological
warfare agents.
4.1 What is Special?
Biological agents
(B-agents) generally considered as suitable for biological warfare include viruses, bacteria, toxins, and
fungi, but only the use of a biological agent in combination with a system
for the deliberate dissemination turns the B-agent into a biological weapon
(B-weapon) or biological warfare agent (BW-agent).
Not all pathogenic microorganisms and their
toxic products are considered as potential biological warfare agents. First,
certain properties and characteristics make them attractive for abuse.
Compared to chemical agents or nuclear
materials, biological agents are unique in their diversity. For technical and
medical B-protection, their classification is important with reference to
detection, identification, prophylaxis, and treatment.
4 Types of Biological Agents
B-agents like bacteria,
viruses, toxins, and fungi differ significantly in size. While
most agents can be identified by microscopic techniques, biological toxins are
at the molecular level and well below the resolution of an electron microscope
(EM).
1 Bacteria
Bacteria are single-cell
microscopic organisms that possess cell walls and cell membranes where the cell
wall maintains the shape. Two different types of cell wall in bacteria exist,
called Gram-positive and Gram-negative. The names originate from
the reaction of cells to the Gram-staining method, a basic test long-employed
for the classification of bacterial species. The difference in Gram reaction of
these two groups of bacteria is thought to be due to a difference in the
structure of their cell walls and explains the distinct susceptibility to
antimicrobial agents as well as the different resistance to physical
disruption, drying, and sodium azide.
The Gram-positive cell wall consists of thick
multilayered peptidoglycan and does not possess a lipid outer membrane. In
contrast, Gram-negative cell walls have only one or a few layers of
peptidoglycan but also possess an outer membrane consisting of various lipid
complexes that confine the perisplasmatic space to the outer milieu.
The difference in cell-wall structure does
not refer to the electrical charge of the bacteria. Both Gram-negative and
Gram-positive bacteria are negatively charged.
Usually, bacteria have a deoxyribonucleic
acid (DNA) genome, transcribe their necessary genes for reproduction and
metabolism to ribonucleic acid (RNA), and translate them to proteins. Bacteria
are generally much smaller than eukaryotic cells but despite their
small size (0.2–5 µm) they are very complex in physiology and cell biology.
Most bacteria replicate by cell division and
grow under simple, well-defined conditions. They are either spherical or
rod-shaped and occur nearly everywhere in nature. Classical examples of
bacteria with potential as BWAs (biological warfare agents) are the species Bacillus anthracis ,Yersinia pestis,
and Francisella tularensis,
and also Coxiella burnetii and Brucella
melitensis. Moreover, some Gram-positive bacteria, like bacilli or
clostridia, produce endospores , a kind of highly resistant and
permanent mold, others are selective parasites, produce gelatinous slimes or
capsules – all features and bacterial characteristics that influence the
efficiency of disinfection.
In
the electron microscopic picture spores show a thin outer spore
coat, thick spore coat, thick spore cortex, and an inner spore membrane
surrounding the chromosomal DNA located in the core region. Furthermore, the
spore's chromosomal DNA is protected by small acid-soluble spore proteins
(SASPs). Spores are generally formed in response to nutritional deprivation and
enable, for example, bacilli to survive in an environment without metabolism.
In addition, spores are extremely resistant to chemical and physical stress
4.2.2 Viruses
Viruses are non-cellular infectious agents consisting of a nucleic acid containing the genetic information, a strand of DNA or RNA, surrounded by a protein capsule (capsid). At the simplest level the capsid protects the viral genome from physical and chemical damage as well as from enzymatic damage. Moreover, many viruses have viral envelopes covering the protein capsid. These envelopes typically consist of phospholipids and proteins and derive from the host cell membranes; some envelopes also include viral glycoproteins. In general the outer surface of the virus is responsible for the recognition of the host cell, which normally takes place by binding of a specific virus-attachment protein to a cellular receptor.
Viruses
are much smaller than bacteria, typically submicroscopic in size and are only
visible by EM. Since viruses cannot reproduce independently they are not
considered as living. They replicate using the host's metabolic processes and
cellular components. In sharp contrast to bacteria, virus structures vary
greatly. They come in many shapes that refer to the structure of the viral
capsid and taxonomy and classification are in part based on these distinctions:
polyhedral, helical, or binal. In addition, classification is based on size,
nucleic acid, replication method, and the presence or absence of a viral
envelope.
Examples of viruses with potential as
biological warfare agent are variola major and variola minor virus, Venezuelan
equine encephalitis virus (VEEV), yellow fever virus (YFV), Ebola virus, and
hantavirus.
4.2.3 Toxins
Toxins of biological
origin are not viable and do not reproduce. Therefore, these B-agents are
neither infectious nor contagious. Biological toxins are either simple
metabolic products from microorganisms or are poisons extracted from plants,
marine organisms, snakes, or insects and share many characteristics like the
absence of smell, taste, and color with chemical warfare agents. The structure
of biological toxins varies from relatively simple to very complex polypeptides
or proteins. Toxic substances can be classified according to the physiological
effects they have on the human body – many cause stomach pains, diarrhea, and
vomiting, and also muscle weakness up to respiratory paralysis.
Classical examples of toxins with potential
as BWAs include some of the botulinum toxins produced by the bacteria Clostridium botulinum, ricin
derived from castor beans, various staphylococcal enterotoxins
from Staphylococcus aureus,
and saxitoxin produced by blue green algae. Biotoxins act on the human body as
either cyto- or neurotoxin. While the former causes cellular destruction, the
latter affects the central nervous system.
4.2.4 Fungi
Fungi can be generally divided into microscopic small yeast and molds. While the former are unicellular organisms the latter are filamentous and almost entirely multicellular and produce asexual endospores. Fungi are heterotrophic organisms, which means they derive their energy from another organism, independent of whether the organism is alive or dead. They are typical pathogens that can be weaponized for use against crops to cause diseases and loss of food plants, like rice blast or cereal rust.
An infected leaf has diamond-shaped or
elliptical or spindle-shaped spots with gray or white centers and brown
margins. The spots may merge and lead to a complete drying of the infected
leaf. The infected panicle turns white and dies before being filled with grain.
Fungal pathogens considered of significance
as biological agents against humans are Coccidioides
immitis and Histoplasma capsulatum. Both fungi are dimorphic
and people are infected by inhalation of spores. Infection is characterized by
flu-like symptoms and the cognate disease primarily affects the lungs.
The valuable contributor of the aboved writing: Andre Richardt
