Page 191 - Textbook of Pathology, 6th Edition

by contaminated water, food, soil, environment or from  TABLE 7.1: Methods of Identification of Microorganisms. 175
an animal host (zoonotic infections).
1. BACTERIA
ii) Spread of infection. Microorganisms after entering the i. Gram stain: Most bacteria
body may spread further through the phagocytic cells, blood ii. Acid fast stain: Mycobacteria, Nocardia
vessels and lymphatics. iii. Giemsa: Campylobacteria
iii) Production of toxins. Bacteria liberate toxins which have 2. FUNGI CHAPTER 7
i. Silver stain: Most fungi
effects on cell metabolism. Endotoxins are liberated on lysis ii. Periodic acid-Schiff (PAS): Most fungi
of the bacterial cell while exotoxins are secreted by bacteria iii. Mucicarmine: Cryptococci
and have effects at distant sites too.
3. PARASITES
iv) Virulence of organisms. Many species and strains of i. Giemsa: Malaria, Leishmania
organisms may have varying virulence e.g. the three strains ii. Periodic acid-Schiff: Amoebae
of C. diphtheriae (gravis, intermedius and mitis) produce the iii. Silver stain: Pneumocystis
same diphtherial exotoxin but in different amounts. 4. ALL CLASSES INCLUDING VIRUSES
i. Culture
v) Product of organisms. Some organisms produce enzy- ii. In situ hybridisation
mes that help in spread of infections e.g. hyaluronidase by iii. DNA analysis
Cl. welchii, streptokinase by streptococci, staphylokinase and iv. Polymerase chain reaction (PCR)
coagulase by staphylococci. Infectious and Parasitic Diseases
species identification and drug sensitivity. Generally, the
Factors Relating to Host organism is looked for at the advancing edge of the lesion in
Microorganisms invade human body when defenses are not the section rather than in the necrotic centre (Fig. 7.1).
adequate. These factors include the following:
DISEASES CAUSED BY BACTERIA,
i) Physical barrier. A break in the continuity of the skin
and mucous membranes allows the microorganisms to enter SPIROCHAETES AND MYCOBACTERIA
the body.
In order to gain an upper hand in human host, bacteria must
ii) Chemical barrier. Mucus secretions of the oral cavity and resist early engulfment by neutrophils. They survive and
the alimentary tract and gastric acidity prevent bacterial damage the host in a variety of ways such as by generation
colonisation. of toxins (e.g. gas-forming anaerobes), by forming a slippery

iii) Effective drainage. Natural passages of the hollow organs capsule that resists attachment to macrophages (e.g.
like respiratory, gastrointestinal, urinary and genital system pneumococci), by inhibition of fusion of phagocytic vacuoles
provide a way to drain the excretions effectively. Similarly, with lysosomes (e.g. tubercle bacilli) etc.
ducts of various glands are the conduits of drainage of Table 7.2 provides an abbreviated classification of
secretions. Obstruction in any of these passages promotes bacterial diseases and their etiologic agents. A few common
infection. and important examples amongst these are discussed below.
iv) Immune defense mechanisms. These include the phago- PLAGUE
cytic leucocytes of blood (polymorphs and monocytes),
phagocytes of tissues (mononuclear-phagocyte system) and Plague is caused by Yersinia (Pasteurella) pestis which is a
the immune system as discussed in Chapter 4. small Gram-negative coccobacillus that grows rapidly on
Some of the common diseases produced by pathogenic most culture media. Direct identification of the organism in
microorganisms are discussed below. Each group of tissues is possible by fluorescence antisera methods.
microorganisms discussed here is accompanied by a Table Plague has been a great killer since 14th century and is
listing diseases produced by them. These lists of diseases known to have wiped out populations of cities. However,
are in no way complete but include only important and the modern Europe is plague free, possibly due to
common examples. No attempts will be made to give details widespread use of arsenic as rat poison. Currently, the world
of organisms as that would mean repeating what is given in over, Vietnam and Tanzania have most cases of plague.
the textbooks of Microbiology. Instead, salient clinico- However, an outbreak in Surat in the state of Gujarat in
pathologic aspects of these diseases are highlighted. Western part of India in 1994 alarmed the world once again
that we are not totally free of this dreaded ‘black death’.
Plague is a zoonotic disease and spreads by rodents,
Methods of Identification
primarily by rats, both wild and domestic; others being
The organisms causing infections and parasitic diseases may squirrels and rabbits. Humans are incidental hosts other than
be identified by routine H & E stained sections in many rodents.
instances. However, confirmation in most cases requires Infection to humans occurs by rat-flea or by inhalation.
either application of special staining techniques or is After the organisms enter the bloodstream, they reach the
confirmed by molecular biologic methods (Table 7.1). In draining lymph nodes where, rather than being phago-
addition, culture of lesional tissue should be carried out for cytosed by phagocytic cells, they proliferate rapidly giving