Microbiology: The Microbial World and You

Course Orientation and Evaluation Criteria

Successful completion of the Microbiology and Parasitology course requires a multi-faceted approach to assessment. Performance is evaluated through five primary components: face-to-face class attendance, which accounts for $10\%$ of the final grade; active participation in class discussions through in-session recitation, accounting for $30\%$; the completion of all quizzes, critical thinking questions, and long tests, totaling $30\%$; achieving passing marks on major exams including the Prelim, Midterm, and Final exams, which constitute $30\%$; and the performance and passing of laboratory exams and activities.

Students are encouraged to adopt specific strategies to pass the course, which include reading material, attending all sessions, accomplishing tasks, listening to lectures, asking questions, thinking critically, and praying. Academic grading follows a specific equivalence scale. For example, in a $100$-item exam, a score of $1$ yields an equivalent of $41$, while a score of $70$ equates to $82$. In a smaller $20$-item assessment, a score of $1$ equates to $43$, $10$ equates to $70$, and $20$ equates to $100$. The grading system distinguishes between lecture and laboratory performance, often splitting the final grade as a $50/50$ weighted average.

Historical Development and the Golden Ages of Microbiology

The history of microbiology is categorized into significant periods of discovery. The First Golden Age of Microbiology took place between $1857$ and $1914$. This era began with the work of Louis Pasteur ($1822$-$1895$), who in $1861$ disproved the theory of spontaneous generation—the hypothesis that living organisms could arise from nonliving matter or a "vital force." Using S-shaped flasks that allowed air but excluded microbes, Pasteur proved biogenesis, the hypothesis that living organisms arise only from preexisting life. In $1857$, he showed that microbes are responsible for fermentation, the conversion of sugar to alcohol. This discovery also linked microbial growth to food spoilage, leading to the development of pasteurization in $1864$, where high heat is applied for a short time to kill spoilage bacteria.

Robert Koch ($1843$-$1910$) was another pioneer who, in $1876$, provided proof that a specific bacterium causes anthrax. He established Koch’s postulates, a sequence of experimental steps used to link a specific microbe to a specific disease: the microorganism must be found in all diseased animals but not healthy ones; it must be isolated and grown in pure culture; the isolated microbe must cause the same disease when inoculated into a susceptible animal; and the microbe must be reisolated from the experimentally infected animal. Other milestones include Robert Hooke’s $1665$ report that living things are composed of boxes or cells and Rudolf Virchow’s $1858$ assertion that cells arise from preexisting cells, forming the basis of Cell Theory.

The Second Golden Age ($1940\text{s}$-$1960\text{s}$) and Third Golden Age ($1980\text{s}$-present) saw a surge in molecular genetics and biotechnology. Key achievements include the clinical testing and mass production of penicillin in the $1940\text{s}$ by Fleming, Chain, and Florey; the sequencing of DNA by Sanger and Gilbert; and the discovery of HIV by Barré-Sinoussi and Montagnier in the $2000\text{s}$. Recent Nobel Prizes include the $2015$ award to Youyou Tu for extracting artemisinin to treat malaria.

The Tree of Life and Microbial Taxonomy

Organisms are classified into a hierarchical system established by Linnaeus, known as scientific nomenclature. Each organism receives two names: a Genus (capitalized) and a specific epithet (lowercase). These names are Latinized, italicized or underlined, and may describe the organism or honor a scientist. For example, Staphylococcus aureus describes clustered ($\text{staphylo-}$) cells and golden ($\text{aureus}$) colonies, while Escherichia coli honors Theodor Escherich and describes the habitat in the colon.

Biological life is divided into three domains: Bacteria (true bacteria), Archaea (prokaryotes living in extreme environments), and Eukarya. The Eukarya domain encompasses four kingdoms: Protista (protozoa and algae), Fungi (yeasts and molds), Plantae, and Animalia. Taxonomy traces organisms through levels including Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species. For instance, humans ($\text{Homo sapiens}$) and the protozoan $\text{Paramecium caudatum}$ share the Domain Eukarya but diverge significantly at the Kingdom level.

Characteristics and Diversity of Microorganisms

The microbial world includes several distinct groups. Bacteria are prokaryotes characterized by peptidoglycan cell walls and reproduction via binary fission; they may use organic chemicals, inorganic chemicals, or photosynthesis for energy. Archaea are also prokaryotes but lack peptidoglycan in their cell walls and include methanogens, extreme halophiles (living in high salt), and extreme thermophiles (living in high heat). Fungi are eukaryotes with chitin cell walls that use organic chemicals for energy; they include unicellular yeasts and multicellular molds or mushrooms composed of mycelia and hyphae filaments.

Protozoa are unicellular eukaryotes that absorb or ingest organic chemicals and may be motile via pseudopods, cilia, or flagella. Algae are eukaryotes with cellulose cell walls that use photosynthesis to produce molecular oxygen and organic compounds. Viruses are acellular entities consisting of a DNA or RNA core surrounded by a protein coat—sometimes enclosed in a lipid envelope—that can only replicate within a living host cell. Helminths are multicellular animal parasites, such as parasitic flatworms and roundworms, which have microscopic stages in their life cycles.

The Role of Microbes in Ecosystems and Human Welfare

Microorganisms play vital roles in the global ecosystem. Bacteria and fungi act as saprophytes or decomposers, breaking down dead and decaying organic material into inorganic nutrients like nitrates, phosphates, sulfates, ammonia, and carbon dioxide. In the nitrogen cycle, nitrogen-fixing bacteria on legume roots convert atmospheric nitrogen gas into ammonia, which nitrifying bacteria then turn into nitrites and nitrates to nourish plants. Microbes also form the base of food chains; tiny organisms like bacteria, algae, phytoplankton, and zooplankton are consumed by larger animals, eventually supporting humans at the top of the chain.

In human welfare, microbes are utilized in bioremediation, a process where genetically engineered microbes decompose industrial wastes, oil, and mercury. Biotechnology uses living organisms to produce foods (vinegar, cheese, bread, kimchi, sausage), drugs, and vaccines. Modern genetic engineering allows bacteria and fungi to produce proteins like insulin, and gene therapy involves replacing missing or defective genes in human cells. Bacteria also serve as biological insecticides, such as Bacillus thuringiensis, which is fatal to insects but harmless to humans and plants.

Pathogenesis, Toxin Production, and Disease Classification

Microorganisms are categorized as pathogens (disease-causing) or non-pathogens. Indigenous microflora (normal microbiota) are microbes that live in or on the human body and are typically beneficial, preventing the growth of pathogens and producing growth factors like folic acid and vitamin K. Resistance is the body’s ability to ward off disease through factors like skin, stomach acid, and antimicrobial chemicals. However, opportunistic pathogens may cause disease if they colonize specific body sites or if the host's resistance is low.

Diseases caused by pathogens fall into two categories: infectious diseases and microbial intoxications. An infectious disease occurs when a pathogen colonizes the body and subsequently causes disease (e.g., MRSA, gas gangrene). A microbial intoxication results from the ingestion of a toxin or poisonous substance produced by a microbe in vitro, or outside the body (e.g., Staphylococcal food poisoning, foodborne botulism). Pathogens cause diverse infections: bacteria are responsible for anthrax, cholera, gonorrhea, and tuberculosis; fungi cause allergies, thrush, and ringworm; protozoa cause malaria and amebic dysentery; and viruses lead to AIDS, influenza, and COVID-19.

Medical Interventions: Vaccination, Asepsis, and Chemotherapy

The management of microbial diseases involves several medical strategies. In $1796$, Edward Jenner developed the first vaccination by inoculating a person with cowpox virus to provide immunity against smallpox. In the $1840\text{s}$, Ignaz Semmelweis advocated handwashing to prevent the transmission of puerperal fever, and in the $1860\text{s}$, Joseph Lister utilized chemical disinfectants (phenol) to prevent surgical wound infections.

Modern chemotherapy involves treating diseases with chemicals, which can be synthetic drugs or antibiotics—chemicals produced by bacteria or fungi that inhibit or kill other microbes. Paul Ehrlich developed the synthetic arsenic drug salvarsan for syphilis in $1910$, and Alexander Fleming discovered the first antibiotic, penicillin, in $1928$ after observing that the fungus Penicillium killed $\text{Staphylococcus aureus}$. Other classes of medications include antibacterial, antifungal, antiprotozoal, and antiviral agents. Some microbes have developed drug resistance, necessitating new strategies and the use of empiric therapy, where treatment is started based on clinical observation before laboratory results are finalized.

Epidemiology and Emerging Infectious Diseases

Epidemiology is the study of the frequency, distribution, and determinants of health and disease in populations. Major causes of death globally include heart disease, stroke, and chronic lung disease. In $2020$, COVID-19 became a leading cause of death in the United States, comparable to heart disease and cancer. Emerging Infectious Diseases (EIDs) are new diseases or diseases increasing in incidence. Examples include West Nile Virus, first diagnosed in Uganda in $1937$ and New York in $1999$; Bovine Spongiform Encephalopathy (BSE), caused by a prion; and Ebola hemorrhagic fever, which causes fever and hemorrhaging.

Other notable EIDs include toxin-producing $\text{Escherichia coli O157:H7}$, a leading cause of diarrhea first seen in $1982$; Invasive group A $\text{Streptococcus}$, which causes extensive tissue damage; and Hantavirus pulmonary syndrome, identified in the U.S. in $1995$. The global AIDS pandemic, caused by HIV and first identified in $1981$, infects approximately $40,000,000$ people worldwide with $14,000$ new infections daily. Anthrax remained a public health concern when $\text{Bacillus anthracis}$ was disseminated via mail in $2001$, infecting $22$ people.

Professional Specializations in Microbiology

The field of microbiology offers various career paths, each focusing on specific organisms or applications. A bacteriologist specializes in the study of bacteria, while a mycologist focuses on fungi. Phycologists study algae, and protozoologists study protozoa. Virologists focus on viruses and their replication. Specialized fields include Immunology, which studies the immune response to invading microbes; Public Health Microbiology and Epidemiology, which monitor and control the spread of diseases in communities; and Industrial Microbiology, which uses microbes to produce vaccines, vitamins, and enzymes in large quantities.