Encyclopedia of Environmental Science and Engineering, Volume I and II

684

MICROBIOLOGY

INTRODUCTION

Microbiology is the study of organisms which are small

enough to require the aid of a microscope to be seen. In a

few cases, organisms are included in this group which can

be seen by the unaided eye because these organisms are

clearly related to the smaller ones. Microorganisms include

viruses, bacteria including rickettsia, mycoplasma, fungi

(yeast and molds), most algae, protozoa and, if one inter-

prets “micro” broadly, certain tiny multicellular plants and

animals. The study of cells and tissues from higher plants

and animals ( tissue culture ) uses techniques common to the

microbiologist and is frequently considered part of modern

microbiology.

Cells in general vary greatly in size but have many simi-

larities in internal organization. Among the most primitive type

of cells, it is impossible to clearly distinguish whether they are

distinctly “plants” or “animals” since they may have some of

the properties of each type. Viruses, on the other hand, are not

cells at all. Instead of arguing endlessly about whether a micro-

organisms is more plant-like or more animal-like and worrying

how to assign viruses, many scientists have divided organisms

in general into those which have (1) only animal characteristics,

(2) only plant characteristics and (3) the Protista (Table 1),

which have some characteristics of both plants and animals.

Some protists, viruses, may have characteristics not shared by

either plants or animals, that is, crystallizability and ability

to reproduce only by infecting some cell and using the cell’s

manufacturing machinery.

PHYSICAL CHARACTERISTICS OF

MICROORGANISMS

Protists vary greatly in size, shape and internal architecture.

Protists are subdivided into prokaryotes, and eukaryotes.

Prokaryotes do not have their genetic material (chromo-

somes) separated from the rest of the cell by a membrane

whereas eukaryotes have a true nucleus ( eu —true, karyo —

nucleus) separated from the rest of the cell by a nuclear

membrane. Viruses (virions) are usually included among the

prokaryotes. There are 9 types of prokaryotes.

Prokaryotes

1) Viruses are the smallest protists. They range in

size from about 30–300 nm. The smallest viruses

can only be visualized with an electron micro-

scope while the largest can be seen with a light

microscope. Viruses are composed of two general

molecular types (1) only one nucleic acid, either

ribonucleic acid (RNA) or deoxyribonucleic acid

(DNA), and (2) a group of proteins also called pro-

tein subunits or capsomeres, which surround the

TABLE 1

Characteristics of the Protista

Virion (virus) 30–300 nm icosahedron, hollow cylinder

icosahedral head 
 tail

RNA, DNA requires participation of host

machinery

Mycoplasma 100–300 nm pleomorphic prokaryotes DNA fission

True bacteria 250–3000 nm spherical, rod, spiral rods,

prokaryotes

DNA fission

Higher bacteria 500–5000 nm spherical, rod, spiral rods,

filamentous, prokaryotes

DNA fission, budding

Prokaryotic algae 500–5000 nm spherical, rods in chains, spiral

rods in chains

DNA fission, internal septation,

gonidia

Eukaryotic algae 500 nm to macroscopic unicellular or multicellular,

filamentous, leafy

DNA in nucleus,

chloroplasts,

mitochondria

asexual or sexual simple fission

to complex life cycles

Protozoa 500–500,000 nm unicellular or colonial various

forms

DNA in nucleus,

mitochondria

asexual or sexual simple fission

to complex life cycles

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