Viruses
Believed to be a bridge between the living and the nonliving,
viruses have either DNA or RNA, seldom both. Their genome is surrounded by a protein
coat, called capsid. Certain
viruses have an envelope, often derived from the host cell membrane during lysis and release.
Multiple characteristics, including type of nucleic acid, single or double strands,
and presence or absence of envelope, are taken into account
in the classification of viruses.
Bacteria
Bacteria are prokaryotes, which means they lack nuclear
membranes and other membrane-bound organelles,
such as mitochondria, Golgi apparatuses, and endoplas- mic
reticula. Most bacteria have a cell wall, but some are devoid of a cell wall
(e.g., Mycoplasma spp.). Bacteria are divided into two major groups,
cocci and bacilli. Cocci (singular coccus)
are spherical and may occur
as single coccus,
as a pair called
diplococcus as in the case of Neisseria gonorrhoeae, as a cluster as seen in the case of Staphylococcus
aureus, or a chain of several
cocci as in the Streptococcus spp. The bacilli (singular
bacillus) are rod-shaped bacteria and they are often referred to as “rods.” They exhibit a considerable variation in their size and shape. Some are
straight rods, other slightly curved, and some are comma-shaped vibrios as in
the case of Vibrio cholerae. Another variation
in the shape is represented by the spiral bacteria called Spirochetes (e.g., Treponema
pallidum, the causal agent of syphilis). Certain “evolved” forms of bacilli tend to have rudimentary filaments, as in the case
of Corynebacterium and
Mycobacterium, and others have a fully developed
filament with true branching, as seen in the case of Streptomyces.
Based on their reactions to Gram staining (color), both cocci and bacilli are
further divided into two groups, Gram-positive
(stain purple) and Gram-negative (stain red). The Gram-positive bac-
terial cell wall is made of a thick layer
of peptidoglycan with some embedded
teichoic acid. The outer layer of the Gram-negative bacterial cell wall is made of a thick layer of lipopolysaccharide, some
phospholipids, and a small amount of peptidoglycan. Both groups of bacteria
have aerobic (oxygen
dependent) and anaerobic
(oxygen independent) members.
There are also several bacteria
that can grow under either condition and they are called
facultative anaerobes. For a detailed discussion on bacterial taxonomy, readers are referred to Bergey’s
Manual of Determinative Bac- teriology and numerous
other authoritative sources
listed in the bibliography.
Fungi
Unlike bacteria, fungi are eukaryotes, which means they have
nuclear membranes and membrane-bound organelle. Almost all have a cell wall, which is usually made
of chitin. Based on their sexual reproduction and other structural features,
fungi are divided into four major groups called divisions
or phylum. Division
Zygomycota can reproduce
sexually and asexually and most (but not all) have no septum in their mycelium. Mucor
and Rhizopus spp.
are examples of zygomycetes. The second divi- sion, Ascomycota, is characterized by the production of ascospores during their sexual reproduction. The ascospores are generally housed in an enclosed structure
called an ascus. An example of an ascomycete
is Pseudallescheria boydii. The third division, Basidiomycota, produce
special cells called
basidia during their
sexual reproduction. The sexual spores, called basidiospores, develop
on the basidia. An example of a
basidiomycete is Filobasidiella neoformans, which is the sexual stage of an important human pathogen, Cryptococcus neoformans. Common mushrooms are also members of division Basidiomycota. Fungi
which are not known to reproduce sexually are called Fungi imperfecti (fourth
division). They multiply vegetatively, but show con- siderable variation
in their structure. As their sexual stages are discovered, they are
generally categorized as either
Ascomycota or Basidiomycota. A great majority of common
airborne fungi, including members of the genus Aspergillus, Alternaria, and Penicillium are examples
of Fungi imperfecti. Many fungi are also known
to produce powerful toxins,
such as aflatoxins, ochratoxins, aminitins, and ergot alkaloids.
Protozoa and Multicellular Parasites
Protozoa are unicellular eukaryotic microorganisms
that belong to kingdom Protista. Protozoa and multicellular
parasites called helminths lack a cell wall. Classification of protozoa and
helminths is complex and the system is not without controversies. Therefore,
the authors have chosen a simple and practical approach which is sum- marized
in the respective chapters on unicellular parasites and multicellular
parasites.
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