Complete eradication of microbial pathogens: Is it possible?
The efforts taken by us to eliminate the pathogens or rather diseases have been remarkable. To summarize, we are able to detect pathogens in laboratory by using traditional cultivation and identification methods as well as ultramodern molecular techniques like PCR. We are aware of causative agents of diseases, their mode of action; importance of hygiene and other prophylactic measures. In the past, much therapeutics like antibiotics and effective vaccines were developed to fight the fatal infections like small pox and plague. Methods like pasteurization are being used daily by housewives to get rid of harmful microbes in the food and hospital facilities to treat our infections. But the emergence of new infections, reemergence of old diseases and their pandemic or epidemics are still the serious cause of concern. In spite of research, training, surveillance, diagnostic facilities and development of novel drugs; the microbial pathogens pose a critical concern not only to physicians and microbiologists but also to economists, policy makers and public health departments. Novel waterborne, airborne, soilborne or zoonotic infections are equally threatening like tuberculosis, pneumonia and flu. We do not have effective medicines against pathogens of dental caries or common cold. We are still fighting to control malaria and our medical safeguards are not efficient enough against HIV infections. Some causes are discussed below which have been major hindrances in eradication strategies of microbial pathogens.
Virulence, pathogenicity and infection: The virulent strains cause disease and are said to be pathogenic (ability to cause disease). Pathogenic microorganisms or infectious agents of humans, animals and plants are of different types. They are classified as Prions (Mad cow disease, Scrapie), Viruses (Measles, AIDS), Bacteria (Tetanus, Cholera), Yeasts (Candidiasis, Cryptococcosis), Fungi (Black piedra, Athlete's foot) and Protozoa (Malaria, Amoebiasis). Major diseases caused by these groups are written in parentheses. All virulent strains possess one or more special properties that contribute to their pathogenicity. These special properties confer adaptability to establish relationship with the host and are perquisite for infection to occur. The process of infection involves invasion, colonization and proliferation of pathogen into the host tissue. Invasion depends on pathogen being able to grow and survive in host tissue. This is achieved by successful attachment to host cells with the help of colonization factors. Colonization factors such as adhesins are specialized structures on cell surface of pathogen, bacteria particularly; that bind to receptors sites on host cells. They adhere to mucosal cell membranes of respiratory, intestinal or genitourinary tracts and therefore pathogen is prevented from washing away by mucosal fluids. Adhesins like capsule, outer membrane proteins, lipopolysaccharide layer, pili or fimbriae in bacteria help in attachment to the host cells during the process of infection. They also protect pathogen from phagocytosis and elimination by host cells. Another important characteristic of pathogen is its ability to compete for nutrients with host cells and host normal flora. Pathogenic bacteria usually produce proteins or chelator compounds like siderophores to sequester iron under iron limiting conditions and access the essential iron required for their growth. Similarly, pathogen also competes for other essential nutrients within host system. Some bacteria also produce toxins and toxigenicity contributes to pathogenicity. Diseases like TSS (Toxic shock syndrome) by Staphylococcus aureus, botulism or food poisoning caused by Clostridium botulinum are examples of toxin induced diseases. Infection by a pathogen often develops characteristic disease symptoms (symptomatic) or asymptomatic (no visible symptoms) or hypersensitive response (allergy) in host. The diseases in plants and animals are caused by similar strategies as in human infections. Thus pathogens have many special features that help to overcome host defense barriers, survival and multiplication in host and as a result contribute to their pathogenicity.
Opportunistic pathogens: Not only pathogenic microorganisms but opportunistic pathogens also compound and interfere in elimination strategy of pathogens. Opportunists are actually present as normal flora but become pathogenic when host immunity is lowered. They often are threatening causative agents of nosocomial infections. Because of opportunistic infections, costs of disease management and treatment measures in hospitals have increased and are of very serious concern.
Multiple drug resistance: Irrational use of antimicrobial agents is the principle reason for the development of multidrug resistant human and animal pathogens. Similar case happened in indiscriminate use of pesticides or bactericides against phytopathogens like Pseudomonas syringae resulting in severe crop losses. Vital pathogens, Staphylococcus aureus, Mycobacterium tuberculosis, Neisseria gonorrheae, Salmonella typhi or Candida albicans and Pneumocystis carinii are multidrug resistant. They have developed multiple drug resistance via production of β-lactamase and penicillinase enzymes or chromosomal mutation. They are non-susceptible to action of effective antibiotics like penicillins and resulted in increased health care cost and worrisome therapeutic failures. Now the infections as common as common cold or ear ache are difficult to treat because of antibiotic resistant pathogens.
Usefulness of pathogens: We did not leave a chance to exploit pathogenic strains for our benefits. Pathogens are best research models to study evolution, adaptation, virulence factors, gene transfer, immunogenicity and progress of infection. Pathogens are recommended for control of arthropod vectors like mosquito and tse-tse fly. The use of pathogenic bacteria like Salmonella has been proposed for prevention of cancer tumors. Scientists are also exploiting pathovars of Escherichia coli for production of vitamins.
Complete elimination of the pathogenic species would never be accomplished; their microscopic size, rapid life cycle, large population, versatile metabolism, genetic diversity and adaptability are far more superior to our eradication strategies.
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