Only $35.99/year

Chapter 1 - The Microbial World and You

Terms in this set (70)

Learning Objectives
1-5 Explain the importance of observations made by Hooke and van Leeuwenhoek.
1-6 Compare spontaneous generation and biogenesis.
1-7 Identify the contributions to microbiology made by Needham, Spallanzani, Virchow, and Pasteur.
1-8 Explain how Pasteur's work influenced Lister and Koch.
1-9 Identify the importance of Koch's postulates.
1-10 Identify the importance of Jenner's work.
1-11 Identify the contributions to microbiology made by Ehrlich and Fleming.
1-12 Define bacteriology, mycology, parasitology, immunology, and virology.
1-13 Explain the importance of microbial genetics and molecular biology.

Bacterial ancestors were the first living cells to appear on Earth. For most of human history, people knew little about the true causes, transmission, and effective treatment of disease. Let's look now at some key developments in microbiology that have spurred the field to its current technological state.

Check Your Understanding
✓ What is the cell theory? 1-5
Check Your Understanding
✓ What evidence supported spontaneous generation? 1-6
✓ How was spontaneous generation disproved? 1-7
Check Your Understanding
✓ Summarize in your own words the germ theory of disease. 1-8
✓ What is the importance of Koch's postulates? 1-9
✓ What is the significance of Jenner's discovery? 1-10
Check Your Understanding
✓ What was Ehrlich's "magic bullet"? 1-11
Check Your Understanding
✓ Define bacteriology, mycology, parasitology, immunology, and virology. 1-12
✓ Differentiate microbial genetics from molecular biology. 1-13
The Germ Theory of Disease - Agostino Bassi (1835) and Pasteur (1865)
• Before the time of Pasteur, effective treatments for many diseases were discovered by trial and error, but the causes of the diseases were unknown.
o The realization that yeasts play a crucial role in fermentation was the first link between the activity of a microorganism and physical and chemical changes in organic materials.
o This discovery alerted scientists to the possibility that microorganisms might have similar relationships with plants and animals—specifically, that microorganisms might cause disease.
o This idea was known as the germ theory of disease.
• The germ theory met great resistance at first because for centuries disease was believed to be punishment for an individual's crimes or misdeeds.
o When the inhabitants of an entire village became ill, people often blamed the disease on demons appearing as foul odors from sewage or on poisonous vapors from swamps.
o Most people born in Pasteur's time found it inconceivable that "invisible" microbes could travel through the air to infect plants and animals or remain on clothing and bedding to be transmitted from one person to another.
o Despite these doubts, scientists gradually accumulated the information needed to support the new germ theory.
• In 1865, Pasteur was called upon to help fight silkworm disease, which was ruining the silk industry in Europe.
o Decades earlier amateur microscopist Agostino Bassi had proved that another silkworm disease was caused by a fungus.
o Using data provided by Bassi, Pasteur found that the more recent infection was caused by a protozoan, and he developed a method for recognizing afflicted silkworm moths.
Modern Developments in Microbiology
• The quest to solve drug resistance, identify viruses, and develop vaccines requires sophisticated research techniques and correlated studies that were never dreamed of in the days of Koch and Pasteur.
• The groundwork laid during the Golden Age of Microbiology provided the basis for several monumental achievements in the years following (Table 1.2).
o New branches of microbiology were developed, including immunology and virology. Most recently, the development of a set of new methods called recombinant DNA technology has revolutionized research and practical applications in all areas of microbiology.

Bacteriology, Mycology, and Parasitology

Bacteriology, the study of bacteria, began with van Leeuwenhoek's first examination of tooth scrapings.
• New pathogenic bacteria are still discovered regularly. Many bacteriologists, like Pasteur, look at the roles of bacteria in food and the environment. One intriguing discovery came in 1997, when Heide Schulz discovered a bacterium large enough to be seen with the unaided eye (0.2 mm wide). This bacterium, named Thiomargarita namibiensis (THĪ-ō-mar-garʹē-tah nahʹmib-ē-EN-sis), lives in the mud on the African coast. Thiomargarita is unusual because of its size and its ecological niche. The bacterium consumes hydrogen sulfide, which would be toxic to mud-dwelling animals (Figure 11.28, page 315).

Mycology, the study of fungi, includes medical, agricultural, and ecological branches.
• Fungal infection rates have been rising during the past decade, accounting for 10% of hospital-acquired infections. Climatic and environmental changes (severe drought) are thought to account for the tenfold increase in Coccidioides immitis (KOK-sid-ē-oi-dēz IM-mi-tis) infections in California. New techniques for diagnosing and treating fungal infections are currently being investigated.

Parasitology is the study of protozoa and parasitic worms.
• Because many parasitic worms are large enough to be seen with the unaided eye, they have been known for thousands of years. It has been speculated that the medical symbol, the rod of Asclepius, represents the removal of parasitic guinea worms (Figure 1.6). Asclepius was a Greek physician who practiced about 1200 B.C. and was deified as the god of medicine.

• The clearing of rain forests has exposed laborers to previously undiscovered parasites. Parasitic diseases unknown until recently are also being found in patients whose immune systems have been suppressed by organ transplants, cancer chemotherapy, or AIDS.
Recombinant DNA Technology
• Microorganisms can now be genetically modified to manufacture large amounts of human hormones and other urgently needed medical substances.
• In the late 1960s, Paul Berg showed that fragments of human or animal DNA (genes) that code for important proteins can be attached to bacterial DNA.
• The resulting hybrid was the first example of recombinant DNA.
• Recombinant DNA (rDNA) technology inserts recombinant DNA into bacteria (or other microbes) to make large quantities of a desired protein.
• This field combines elements from two other areas of study, including microbial genetics, which studies the mechanisms by which microorganisms inherit traits, and molecular biology, which looks at how genetic information is carried in molecules of DNA and how DNA directs the synthesis of proteins.

• Although molecular biology encompasses all organisms, much of our knowledge of how genes determine specific traits has been revealed through experiments with bacteria.
• Unicellular organisms, primarily bacteria, have several advantages for genetic and biochemical research.
• Bacteria are less complex than plants and animals, and the life cycles of many bacteria last less than an hour, so scientists can cultivate very large numbers of bacteria for study in a relatively short time.

• Once science turned to the study of unicellular life, rapid progress was made in genetics.
• In the 1940s, George W. Beadle and Edward L. Tatum demonstrated the relationship between genes and enzymes; DNA was established as the hereditary material by Oswald Avery, Colin MacLeod, and Maclyn McCarty; and Joshua Lederberg and Edward L. Tatum discovered that genetic material could be transferred from one bacterium to another by a process called conjugation.
• Then in the 1950s, James Watson and Francis Crick proposed a model for the structure and replication of DNA.
• The early 1960s also witnessed a further explosion of discoveries relating to the way DNA controls protein synthesis.
• François Jacob and Jacques Monod discovered messenger RNA (ribonucleic acid), a chemical involved in protein synthesis, and later they made the first major discoveries about the regulation of gene function in bacteria.
• During the same period, scientists were able to break the genetic code and thus understand how the information for protein synthesis in messenger RNA is translated into the amino acid sequence for making proteins.
Microbes and Human Welfare
• As mentioned earlier, only a minority of all microorganisms are pathogenic.
• Microbes that cause food spoilage, such as soft spots on fruits and vegetables, decomposition of meats, and rancidity of fats and oils, are also a minority.
• The vast majority of microbes benefit humans, other animals, and plants in many ways.
• For example, microbes produce methane and ethanol that can be used as alternative fuels to generate electricity and power vehicles.
• Biotechnology companies are using bacterial enzymes to break down plant cellulose so that yeast can metabolize the resulting simple sugars and produce ethanol.
• The following sections outline some of these beneficial activities. In later chapters, we will discuss these activities in greater detail.

Recycling Vital Elements
• Discoveries made by two microbiologists in the 1880s have formed the basis for today's understanding of the biogeochemical cycles that support life on Earth.
• Martinus Beijerinck and Sergei Winogradsky were the first to show how bacteria help recycle vital elements between the soil and the atmosphere.
• Microbial ecology, the study of the relationship between microorganisms and their environment, originated with the work of these scientists.
• Today, microbial ecology has branched out and includes the study of how microbial populations interact with plants and animals in various environments.
• Among the concerns of microbial ecologists are water pollution and toxic chemicals in the environment.

• The chemical elements carbon, nitrogen, oxygen, sulfur, and phosphorus are essential for life and abundant, but not necessarily in forms that organisms can use.
• Microorganisms are primarily responsible for converting these elements into forms that plants and animals can use.
• Microorganisms, especially bacteria and fungi, return carbon dioxide to the atmosphere when they decompose organic wastes and dead plants and animals.
• Algae, cyanobacteria, and higher plants use the carbon dioxide during photosynthesis to produce carbohydrates for animals, fungi, and bacteria.
• Nitrogen is abundant in the atmosphere but in that form is not usable by plants and animals.
• Only bacteria can naturally convert atmospheric nitrogen to a form available to plants and animals.
Emerging Infectious Diseases
• These recent outbreaks point to the fact that infectious diseases are not disappearing, but rather seem to be reemerging and increasing. In addition, a number of new diseases—emerging infectious diseases (EIDs)—have cropped up in recent years. These are diseases that are new or changing and are increasing or have the potential to increase in incidence in the near future. Some of the factors that have contributed to the development of EIDs are evolutionary changes in existing organisms (e.g., Vibrio cholerae; VIBrē- ō KOL-er-ī); the spread of known diseases to new geographic regions or populations by modern transportation (e.g., West Nile virus); and increased human exposure to new, unusual infectious agents in areas that are undergoing ecologic changes such as deforestation and construction (e.g., Venezuelan hemorrhagic virus). EIDs also develop as a result of antimicrobial resistance (e.g., vancomycin-resistant S. aureus). An increasing number of incidents in recent years highlights the extent of the problem.

• Between April 2012 and June 2014, there were 339 confirmed cases and 100 deaths in humans caused by a new virus called Middle East respiratory syndrome coronavirus (MERS-CoV). The virus belongs to the same family that causes illnesses from the common cold to severe acute respiratory syndrome (SARS), to be described shortly. Because all reported cases are linked to the Middle East, this latest emerging infectious disease is called Middle East respiratory syndrome (MERS).

• Severe acute respiratory syndrome (SARS) is an emerging infectious disease that first appeared in China in 2002. It is a viral infection caused by the SARS-associated coronavirus (SARS-CoV).

• H1N1 influenza (flu), also known as swine flu, is a type of influenza caused by a new virus called influenza H1N1. H1N1 was first detected in the United States in 2009, and that same year the World Health Organization declared H1N1 flu to be a pandemic disease (a disease that affects large numbers of individuals in a short period of time and occurs worldwide).

• Avian influenza A (H5N1), or bird flu, caught the attention of the public in 2003, when it killed millions of poultry and 24 people in southeast Asia. Avian influenza viruses occur in birds worldwide. In 2013, a different avian influenza, H7N9, sickened 131 people in China.

• Influenza A viruses are found in many different animals, including ducks, chickens, pigs, whales, horses, and seals. Normally, each subtype of influenza A virus is specific to certain species. However, influenza A viruses normally seen in one species sometimes can cross over and cause illness in another species, and all subtypes of influenza A virus can infect pigs. Although it is unusual for people to get influenza infections directly from animals, sporadic human infections and outbreaks caused by certain avian influenza A viruses and pig influenza viruses have been reported. As of 2008, avian influenza had sickened 242 people, and about half of them died. Fortunately, the virus has not yet evolved to be transmitted successfully among humans.

• Human infections with avian influenza viruses detected since 1997 have not resulted in sustained human-to-human transmission. However, because influenza viruses have the potential to change and gain the ability to spread easily between people, monitoring for human infection and person-to-person transmission is important (see the box in Chapter 13 on page 363).

• Antibiotics are critical in treating bacterial infections. However, years of overuse and misuse of these drugs have created environments in which antibiotic-resistant bacteria thrive. Random mutations in bacterial genes can make a bacterium resistant to an antibiotic. In the presence of that antibiotic, this bacterium has an advantage over other, susceptible bacteria and is able to proliferate. Antibiotic-resistant bacteria have become a global health crisis.

• Staphylococcus aureus causes a wide range of human infections from pimples and boils to pneumonia, food poisoning, and surgical wound infections, and it is a significant cause of hospital-associated infections. After penicillin's initial success in treating S. aureus infection, penicillin-resistant S. aureus became a major threat in hospitals in the 1950s, requiring the use of methicillin. In the 1980s, methicillin-resistant S. aureus, called MRSA, emerged and became endemic in many hospitals, leading to increasing use of vancomycin. In the late 1990s, S. aureus infections that were less sensitive to vancomycin (vancomycin-intermediate S. aureus, or VISA) were reported. In 2002, the first infection caused by vancomycin-resistant S. aureus (VRSA) in a patient in the United States was reported.

• In 2010, the World Health Organization (WHO) reported that in some parts of the world (such as northwestern Russia) about 28% of all individuals with tuberculosis (TB) had the multidrug-resistant form of the disease (MDR-TB). Multidrugresistant TB is caused by bacteria that are resistant to at least the antibiotics isoniazid and rifampicin, the most effective drugs against tuberculosis.

• The antibacterial substances added to various household cleaning products are similar to antibiotics in many ways. When used correctly, they inhibit bacterial growth. However, wiping every household surface with these antibacterial agents creates an environment in which the resistant bacteria survive. Unfortunately, when you really need to disinfect your homes and hands—for example, when a family member comes home from a hospital and is still vulnerable to infection—you may encounter mainly resistant bacteria.

• Routine housecleaning and handwashing are necessary, but standard soaps and detergents (without added antibacterials) are fine for these tasks. In addition, quickly evaporating chemicals, such as chlorine bleach, alcohol, ammonia, and hydrogen peroxide, remove potentially pathogenic bacteria but do not leave residues that encourage the growth of resistant bacteria.

• West Nile encephalitis (WNE) is inflammation of the brain caused by West Nile virus (see the Clinical Focus box on page 215). WNE was first diagnosed in the West Nile region of Uganda in 1937. In 1999 the virus made its first North American appearance in humans in New York City. In 2007, West Nile virus infected over 3600 people in 43 states. West Nile virus is now established in nonmigratory birds in 48 states. The virus, which is carried by birds, is transmitted between birds—and to horses and humans—by mosquitoes. West Nile virus may have arrived in the United States in an infected traveler or in migratory birds.

• In 1996, countries worldwide were refusing to import beef from the United Kingdom, where hundreds of thousands of cattle born after 1988 had to be killed because of an epidemic of bovine spongiform encephalopathy (en-sef-a-LOP-a-thē), also called BSE or mad cow disease. BSE first came to the attention of microbiologists in 1986 as one of a handful of diseases caused by an infectious protein called a prion. Studies suggest that the source of disease was cattle feed prepared from sheep infected with their own version of the disease. Cattle are herbivores (plant eaters), but adding protein to their feed improves their growth and health. Creutzfeldt-Jakob disease (KROITS-felt YA-kob), or CJD, is a human disease also caused by a prion. The incidence of CJD in the United Kingdom is similar to the incidence in other countries. However, by 2005 the United Kingdom reported 154 human cases of CJD caused by a new variant related to the bovine disease (see Chapter 22).

• Escherichia coli is a normal inhabitant of the large intestine of vertebrates, including humans, and its presence is beneficial because it helps produce certain vitamins and breaks down otherwise undigestible foodstuffs (see Chapter 25). However, a strain called E. coli O157:H7 causes bloody diarrhea when it grows in the intestines. This strain was first recognized in 1982 and since then has emerged as a public health problem. It is now one of the leading causes of diarrhea worldwide. In 1996, some 9000 people in Japan became ill, and 7 died, as a result of infection by E. coli O157:H7. The recent outbreaks of E. coli O157:H7 in the United States, associated with contamination of undercooked meat and unpasteurized beverages, have led public health officials to call for the development of new methods of testing for bacteria in food.

• In 2004, emergence of a new epidemic strain of Clostridium difficile (klos-TRID-ē-um DIF-fi-sē-il) was reported. The epidemic strain produces more toxins than others and is more resistant to antibiotics. In the United States, C. difficile infections kill nearly 14,000 people a year. Nearly all of the C. difficile infections occur in health care settings, where the infection is frequently transmitted between patients via health care personnel whose hands are contaminated after contact with infected patients or their surrounding environment.

• In 1995, a hospital laboratory technician in Democratic Republic of Congo (DROC) who had fever and bloody diarrhea underwent surgery for a suspected perforated bowel. Afterward he started hemorrhaging, and his blood began clotting in his blood vessels. A few days later, health care workers in the hospital where he was staying developed similar symptoms. One of them was transferred to a hospital in a different city; personnel in the second hospital who cared for this patient also developed symptoms. By the time the epidemic was over, 315 people had contracted Ebola hemorrhagic fever (hem-or-RAJ-ik), or EHF, and over 75% of them died. The epidemic was controlled when microbiologists instituted training on the use of protective equipment and educational measures in the community. Close personal contact with infectious blood or other body fluids or tissue (see Chapter 23) leads to human-to-human transmission.

• Microbiologists first isolated Ebola viruses from humans during earlier outbreaks in DROC in 1976. (The virus is named after the Democratic Republic of the Congo's Ebola River.) In 2014, the World Health organization declared, an Ebola virus outbreak in West Africa. In 1989 and 1996, outbreaks among monkeys imported into the United States from the Philippines were caused by another Ebola virus but were not associated with human disease.

• Recorded cases of Marburg virus, another hemorrhagic fever virus, are rare. The first cases were laboratory workers in Europe who handled African green monkeys from Uganda. Four outbreaks were identified in Africa between 1975 and 1998, involving 2 to 154 people with 56% mortality. In 2004, an outbreak killed 227 people. African fruit bats are the natural reservoir for the Marburg virus, and microbiologists suspect that bats are the reservoir for EHF.

• In 1993, an outbreak of cryptosporidiosis (KRIP-tō-sporiʹdē- Ō-sis) transmitted through the public water supply in Milwaukee, Wisconsin, resulted in diarrheal illness in an estimated 403,000 persons. The microorganism responsible for this outbreak was the protozoan Cryptosporidium (KRIP-tō-sporiʹdē- um). First reported as a cause of human disease in 1976, it is responsible for up to 30% of the diarrheal illness in developing countries. In the United States, transmission has occurred via drinking water, swimming pools, and contaminated hospital supplies.

• AIDS (acquired immunodeficiency syndrome) first came to public attention in 1981 with reports from Los Angeles that a few young homosexual men had died of a previously rare type of pneumonia known as Pneumocystis (noo-mō-SIS-tis) pneumonia. These men had experienced a severe weakening of the immune system, which normally fights infectious diseases. Soon these cases were correlated with an unusual number of occurrences of a rare form of cancer, Kaposi's sarcoma, among young homosexual men. Similar increases in such rare diseases were found among hemophiliacs and intravenous drug users.

• Researchers quickly discovered that the cause of AIDS was a previously unknown virus (see Figure 1.1e). The virus, now called human immunodeficiency virus (HIV), destroys CD4+ T cells, one type of white blood cell important to immune system defenses. Sickness and death result from microorganisms or cancerous cells that might otherwise have been defeated by the body's natural defenses. So far, the disease has been inevitably fatal once symptoms develop.

• By studying disease patterns, medical researchers found that HIV could be spread through sexual intercourse, by contaminated needles, from infected mothers to their newborns via breast milk, and by blood transfusions—in short, by the transmission from one person to another. Since 1985, blood used for transfusions has been carefully checked for the presence of HIV, and it is now quite unlikely that the virus can be spread by this means.

• By the end of 2013, over 1 million people in the United States were living with AIDS. About 50,000 Americans become infected and 18,000 die each year. As of 2011, health officials estimated that 1.8 million Americans have HIV infection. In 2013, the World Health Organization (WHO) estimated that over 35 million people worldwide are living with HIV/AIDS and that 6000 new infections occur every day.

• Since 1994, new treatments have extended the life span of people with AIDS. The majority of individuals with AIDS are in the sexually active age group. Because heterosexual partners of AIDS sufferers are at high risk of infection, public health officials are concerned that even more women and minorities will contract AIDS. In 1997, HIV diagnoses began increasing among women and minorities. Among the AIDS cases reported in 2009, 26% were women, and 49% were African American.

• In the months and years to come, scientists will continue to apply microbiological techniques to help them learn more about the structure of the deadly HIV, how it is transmitted, how it grows in cells and causes disease, how drugs can be directed against it, and whether an effective vaccine can be developed. Public health officials have also focused on prevention through education.

• AIDS poses one of this century's most formidable health threats, but it is not the first serious epidemic of a sexually transmitted infection. Syphilis was also once a fatal epidemic disease. As recently as 1941, syphilis caused an estimated 14,000 deaths per year in the United States. With few drugs available for treatment and no vaccines to prevent it, efforts to control the disease focused mainly on altering sexual behavior and on the use of condoms. The eventual development of drugs to treat syphilis contributed significantly to preventing the spread of the disease. According to the Centers for Disease Control and Prevention (CDC), reported cases of syphilis dropped from a record high of 575,000 in 1943 to an all-time low of 5979 cases in 2004. Since then, however, the number of cases has been increasing.

• Just as microbiological techniques helped researchers in the fight against syphilis and smallpox, they will help scientists discover the causes of new emerging infectious diseases in the twenty-first century. Undoubtedly there will be new diseases. Ebola virus and Influenzavirus are examples of viruses that may be changing their abilities to infect different host species. Emerging infectious diseases will be discussed further in Chapter 14 on page 405.

• Infectious diseases may reemerge because of antibiotic resistance (see the Clinical Focus box in Chapter 26 on page 756) and through the use of microorganisms as weapons. (See the Clinical Focus box in Chapter 23 on page 645.) The breakdown of public health measures for previously controlled infections has resulted in unexpected cases of tuberculosis, whooping cough, and diphtheria (see Chapter 24).