Page 35 - Review of Medical Microbiology and Immunology ( PDFDrive )

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A
C
T
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Classification of Medically

Important Bacteria

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CHAPTER C ONTENT S

Principles of Classification
Pearls

PRINCIPLES OF CLASSIFICATION
genus when DNA sequencing of their genome reveals they
The current classification of bacteria is based primarily on
are significantly different and should be classified in a new
morphologic and biochemical characteristics. A scheme examples of these criteria placing bacteria into the same
or different genus. For example, an organism formerly
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that divides the medically important organisms by genus is
known as Pseudomonas cepacia has been reclassified as
shown in Table 5–1. For pedagogic purposes, this classifica-
Burkholderia cepacia because the base sequence of its DNA
tion scheme deviates from those derived from strict taxo-
nomic principles in two ways:
members of the genus Pseudomonas.
(1) Only organisms that are described in this book in
the section on medically important bacteria are included.
(2) Because there are so many gram-negative rods, they
PEARLS
are divided into three categories: respiratory organisms,
zoonotic organisms, and enteric and related organisms.
The initial criterion used in the classification is the
such as the nature of the cell wall, staining characteristics,
nature of the cell wall (i.e., is it rigid, flexible, or absent?).
ability to grow in the presence or absence of oxygen, and
Bacteria with rigid, thick walls can be subdivided into free- • The classification of bacteria is based on various criteria,
ability to form spores.
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living bacteria, which are capable of growing on laboratory
• The criterion currently used is the base sequence of the
medium in the absence of human or other animal cells, and
genome DNA. Several bacteria have been reclassified on
non–free-living bacteria, which are obligate intracellular
the basis of this information.
parasites and therefore can grow only within human or
other animal cells. The free-living organisms are further
subdivided according to shape and staining reaction into a
variety of gram-positive and gram-negative cocci and rods
with different oxygen requirements and spore-forming
COURSE EXAMINATIONS
abilities. Bacteria with flexible, thin walls (the spirochetes)
and those without cell walls (the mycoplasmas) form sepa-
rate units.
found in the Basic Bacteriology section of Part XIII:
USMLE (National Board) Practice Questions starting on
Using these criteria, along with various biochemical
reactions, many bacteria can be readily classified into sepa- Questions on the topics discussed in this chapter can be
page 709. Also see Part XIV: USMLE (National Board)
Practice Examination starting on page 751.
rate genus and species. However, there have been several
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