Resistance of Microorganisms to Decontamination
Microorganisms differ greatly in their ability to tolerate destruction
by physical or chemical means. As demonstrated in Fig. 1 vegetative
bacteria, viruses, fungi, and mycobacteria are often considered the
least resistant to decontamination and can usually be reduced to a
sanitary level by sanitizers or destroyed by disinfection methods.
Bacterial endospores and other protective shell structures such as
oocysts and eggs are the most resistant type of pathogen and are only
killed by sterilization processes. The potent power of physical or
chemical sterilization processes destroys the robust protective layers
of these endospores and shell structures, destroying their genomes (Lai
et al., 2003; Riesenman & Nicholson, 2000; Setlow, 2006; Swenson,
2012).
Endospores are the most resistant type of pathogen and extreme
sterilization methods are required to destroy them. Endospores are a
pathogen’s method of surviving in extreme conditions. Endospores ofBacillus species have demonstrated the ability to resist and
survive extreme conditions, such as highly acidic environments,
prolonged exposure to high temperatures, non-ionizing and ionizing
radiation, as well as strong antibiotics including ampicillin,
cephalothin and oxacillin (Berg & Grecz, 1970; Byrne, Dunne, & Bolton,
2006; Clavel, Carlin, Lairon, Nguyen-The, & Schmitt, 2004; Schlegelova,
Babak, Brychta, Klimova, & Napravnikova, 2003; Setlow, 1995).
Spores have multiple protective layers, which act as barriers, and
accounts for their extreme resistance to decontamination. The first
barrier is the external layer, which consists of either an exosporium or
a spore coat. The exosporium is composed of a number of different
proteins, while a spore coat consists of both proteins and
glycoproteins. The external layer has the ability to filter and detoxify
many environmental contaminants (Lai et al., 2003; Setlow, 2006). The
barrier beneath the external layer is the cortex, which is formed of a
thick layer of peptidoglycans. The cortex protects the core from
destruction by organic solvents. The third barrier, situated beneath the
cortex, is the cell wall, which is again composed of peptidoglycans.
Beneath the cell wall is a cell membrane, which safeguards the central
core. The final barrier is the central core, which consists of small
acid-soluble binding proteins (SASP) that protect the DNA. Spores are
able to survive for many years until favorable conditions arise, at
which point they can then develop into vegetative cells (Driks, 2002;
Riesenman & Nicholson, 2000; Setlow, 2006).
Protozoa are microscopic unicellular organisms, which are widespread in
almost every habitat. Some species of protozoa are commensal and are not
pathogenic to their hosts, whereas others are pathogenic and may cause a
range of diseases from mild in severity to life-threatening, such as
malaria. Infection from protozoa can be caused by contaminated water,
food, and soil via sporulated oocysts passed in the feces of the host.
Protozoal oocysts, which are an essential stage of the life cycle of
protozoa ((CDC), 2004; Yaeger., 1996) have a high level of resistance to
chemical and physical decontamination treatments, due to their
protective membrane or hardy cell wall that is composed of two layers of
over 90% protien. The outer layer of the oocyst wall contains mainly
lipids-free quinone-tanned proteins, whilst the inner layer consists of
a lipid-protein matrix (Mai et al., 2009).
Helminth cause parasitic infections that lead to the tropical disease,
Helminthiasis. The female helminth worm deposits the eggs into the host
in a process known as oviposition. Adult helminth can deposit up to
700,000 eggs, six times a day. The helminth eggs are highly resistant to
chemical and physical decontamination methods, because of their layered
structure which provides resistance under several conditions. There are
three basic layers which consist of an outer proteinic layer, followed
by a chitinous layer, and then an inner lipoidal layer (Jimenez, 2007).
The persistence of helminth ova is the main constraint for the reuse of
water and wastewater (WHO, 2006).
Fungi and fungal spores exhibit high resistance to decontamination
treatments (Ma & Bibby, 2017). Waterborne fungi are considered
responsible for environmental problems such as turbidity, odor and
mycotoxin emissions, in addition to waterborne diseases caused byAspergillus spp . and Penicillium spp . (Curtis, Lieberman,
Stark, Rea, & Vetter, 2009; Hageskal, Knutsen, Gaustad, de Hoog, &
Skaar, 2006; Oliveira, Barreto Crespo, & Pereira, 2020; Pereira et al.,
2009).
Bacteria can be classified as Gram positive (GP) or Gram negative (GN),
based on the structure of the cell wall. The cell wall of GP bacteria is
characterized by a thick peptidoglycan layer with no outer lipid
membrane, while the peptidoglycan layer is thin in GN bacteria, and
supported with an outer lipid membrane (Gram, 1884). 90-95% of GN
bacteria are pathogenic and are often implicated in severe disease, such
as Cholera caused by Vibrio cholerae , whilst most GP bacteria are
non-pathogenic (Abe et al., 2010; Alexandraki & Palacio, 2010).
Although these pathogenic GN bacteria show more resistance to
antibiotics than GP strains, they are more susceptible to
decontamination methods and can be easily decontaminated. Comparatively,
GP bacteria have more resistance to decontamination methods, but tend to
be less harmful to humans (Howie, Alfa, & Coombs, 2008; Traverse &
Aceto, 2015).
Mycobacteria has its name derived from the latin word myco , which
refers to fungus, because mycobacteria have been observed to grow in a
mold-like manner when cultivated in laboratories. Mycobacteria are
responsible for serious diseases in humans, such as tuberculosis and
leprosy (Ryan & Ray, 2004). Mycobacteria show a high level of
resistance to chemical and physical decontamination methods due to their
cell wall, which is composed of hydrophobic mycolic acid and
peptidoglycan layers that are interconnected by a highly branched
polysaccharide (arabinogalactan), which represents about 80% of the
cell wall (Alderwick, Harrison, Lloyd, & Birch, 2015; Jackson, 2014).
The extracellular form of a virus that spreads from one organism to
another is called a virion. In contrast to other microorganisms, viruses
can not be considered as living organisms as they lack their own
metabolism. A virion consists of a viral genome (containing both DNA and
RNA), which is enclosed in a protein capsid that provides protection to
the genome. Viruses can be classified into two types; enveloped and
non-enveloped, according to their cell membrane. Viruses are referred to
as enveloped when the protein capsid is surrounded by a membrane
(“envelope”), which is composed of a lipid bilayer studded with
virus-coded proteins in the shape of spikes or knobs, called peplomers.
The role of the biological membrane is to protect the virus against
attack from the host immune system. Viruses without a membrane are known
as non-enveloped or “naked” viruses. Contrary to what one would
expect, non-enveloped viruses are the most resistant to decontamination
methods, and smaller non-enveloped viruses are more resistant than
larger ones. This is because outer lipid bilayer “envelopes” can be
easily neutralized by various chemical and physical agents, and a virion
is only infectious if it is fully intact. Hence, if the envelope is
destroyed, a virion is no longer infectious (Gelderblom, 1996; Howie et
al., 2008; Traverse & Aceto, 2015).