Bacterial Diseases in Humans

History of Bacterial Diseases

Infectious diseases, spread from person to person by bacteria, are among the leading causes of death despite advances in medical research.

Learning Objectives

Give examples of historical, new, and re-emerging bacterial diseases in humans

Key Takeaways

Key Points

  • A pathogen must be able to reproduce in the host’s body and damage the host in some way to cause disease.
  • Before antibiotics, contracting plagues usually meant death; however, most bacterium associated with these plagues respond to modern antibiotics; mortality rates from these diseases are now very low.
  • Emerging diseases include those that have appeared in a population for the first time or that may have existed previously, but are rapidly spreading; this also includes re-emerging diseases that were previously under control.
  • The spread of disease can be impacted dramatically by changes in the environment, the pathogen, or the host population.

Key Terms

  • zoonosis: an animal disease that can be transmitted to humans
  • plague: an epidemic or pandemic caused by any pestilence
  • pathogen: any organism or substance, especially a microorganism, capable of causing disease, such as bacteria, viruses, protozoa, or fungi

Long History of Bacterial Disease

There are records about infectious diseases as far back as 3000 B.C.E. A number of significant pandemics caused by bacteria have been documented over several hundred years. Some of the most memorable pandemics led to the decline of cities and nations.

In the 21st century, infectious diseases remain among the leading causes of death worldwide, despite advances made in medical research and treatments in recent decades. A disease spreads when the pathogen that causes it is passed from one person to another. For a pathogen to cause disease, it must be able to reproduce in the host’s body and damage the host in some way.

The Plague of Athens

In 430 B.C.E., the Plague of Athens killed one-quarter of the Athenian troops that were fighting in the great Peloponnesian War and weakened Athens’ dominance and power. The plague impacted people living in overcrowded Athens as well as troops aboard ships that had to return to Athens. The source of the plague may have been identified recently when researchers from the University of Athens were able to use DNA from teeth recovered from a mass grave. The scientists identified nucleotide sequences from a pathogenic bacterium, Salmonella enterica serovar typhi, which causes typhoid fever. This disease is commonly seen in overcrowded areas and has caused epidemics throughout recorded history.

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Salmonella enterica serovar typhi: Salmonella enterica serovar typhi, the causative agent of typhoid fever, is a gram-negative, rod-shaped gamma protobacterium. Typhoid fever, which is spread through feces, causes intestinal hemorrhage, high fever, delirium and dehydration. Today, between 16 and 33 million cases of this re-emerging disease occur annually, resulting in over 200,000 deaths. Carriers of the disease can be asymptomatic. In a famous case in the early 1900s, a cook named Mary Mallon unknowingly spread the disease to over fifty people, three of whom died. Other Salmonella serotypes cause food poisoning.

Bubonic Plagues

From 541 to 750 C.E.., an outbreak of what was likely a bubonic plague (the Plague of Justinian), eliminated one-quarter to one-half of the human population in the eastern Mediterranean region. The population in Europe dropped by 50 percent during this outbreak. The bubonic plague would strike Europe more than once.

One of the most devastating pandemics was the Black Death (1346 to 1361) that is believed to have been another outbreak of bubonic plague caused by the bacterium Yersinia pestis. It is thought to have originated initially in China and spread along the Silk Road, a network of land and sea trade routes, to the Mediterranean region and Europe, carried by rat fleas living on black rats that were always present on ships. The Black Death reduced the world’s population from an estimated 450 million to about 350 to 375 million. Bubonic plague struck London hard again in the mid-1600s. In modern times, approximately 1,000 to 3,000 cases of plague arise globally each year. Although contracting bubonic plague before antibiotics meant almost certain death, the bacterium responds to several types of modern antibiotics; mortality rates from plague are now very low.

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Bubonic plague: The (a) Great Plague of London killed an estimated 200,000 people, or about twenty percent of the city’s population. The causative agent, the (b) bacterium Yersinia pestis, is a gram-negative, rod-shaped bacterium from the class Gamma Proteobacteria. The disease is transmitted through the bite of an infected flea, which is infected by a rodent. Symptoms include swollen lymph nodes, fever, seizure, vomiting of blood, and (c) gangrene.

Migration of Diseases to New Populations

Over the centuries, Europeans tended to develop genetic immunity to endemic infectious diseases, but when European conquerors reached the western hemisphere, they brought with them disease-causing bacteria and viruses, which triggered epidemics that completely devastated populations of Native Americans who had no natural resistance to many European diseases. It has been estimated that up to 90 percent of Native Americans died from infectious diseases after the arrival of Europeans, making conquest of the New World a foregone conclusion.

Emerging and Re-emerging Diseases

The distribution of a particular disease is dynamic. Therefore, changes in the environment, the pathogen, or the host population can dramatically impact the spread of a disease. According to the World Health Organization (WHO), an emerging disease is one that has appeared in a population for the first time, or that may have existed previously, but is rapidly increasing in incidence or geographic range. This definition also includes re-emerging diseases that were previously under control. Approximately 75 percent of recently-emerging infectious diseases affecting humans are zoonotic diseases. Zoonoses, diseases that primarily infect animals and are transmitted to humans, are of both viral and bacterial origins. Brucellosis is an example of a prokaryotic zoonosis that is re-emerging in some regions. Necrotizing fasciitis (commonly known as flesh-eating bacteria) has been increasing in virulence for the last 80 years, for unknown reasons.

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Regions of bacterial disease emergence: The map shows regions where bacterial diseases are emerging or reemerging.

Some of the currently-emerging diseases are not actually new, but are diseases that were catastrophic in the past. They devastated populations, became dormant for a while, but have re-emerged, sometimes more virulent than before. Such was the case with bubonic plague. Other diseases, like tuberculosis, were never eradicated, but were under control in some regions of the world until re-emerging, mostly in urban centers with high concentrations of immunocompromised people. The WHO has identified certain diseases whose worldwide re-emergence should be monitored. Among these are two viral diseases (dengue fever and yellow fever) and three bacterial diseases (diphtheria, cholera, and bubonic plague). The war against infectious diseases has no foreseeable end.

Biofilms and Disease

Biofilms, complex colonies of bacteria acting as a unit in their release of toxins, are highly resistant to antibiotics and host defense.

Learning Objectives

Give examples of the roles played by biofilms in human diseases

Key Takeaways

Key Points

  • Once a biofilm infection is established, it is very difficult to eradicate because biofilms exhibit great resistance to most methods used to control microbial growth, including antibiotics.
  • Biofilms are able to grow anywhere there is an optimal combination of moisture, nutrients, and a surface.
  • Biofilms are responsible for diseases such as infections in patients and readily settle within wounds and burns; they can also easily colonize medical devices and other surfaces where sterility is vital for health.

Key Terms

  • biofilm: a thin film of mucus created by and containing a colony of bacteria and other microorganisms
  • nosocomial: contracted in a hospital, or arising from hospital treatment

Biofilms and Disease

Biofilms are complex colonies of bacteria (often containing several species) that exchange chemical signals to coordinate the release of toxins that will attack the host. Once established, they are very difficult to destroy as they are highly resistant to antimicrobial treatments and host defense. Biofilms form when microorganisms adhere to the surface of some object in a moist environment and begin to reproduce. They grow virtually everywhere in almost any environment where there is a combination of moisture, nutrients, and a surface. Biofilms are responsible for diseases such as infections in patients with cystic fibrosis, Legionnaires’ disease, and otitis media. They produce dental plaque and colonize catheters, prostheses, transcutaneous and orthopedic devices, contact lenses, and internal devices such as pacemakers. They also form in open wounds and burned tissue. In healthcare environments, biofilms grow on hemodialysis machines, mechanical ventilators, shunts, and other medical equipment. In fact, 65 percent of all infections acquired in the hospital (nosocomial infections) are attributed to biofilms. Biofilms are also related to diseases contracted from food because they colonize the surfaces of vegetable leaves and meat, as well as food-processing equipment that is not adequately cleaned.

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The Five Stages of Biofilm Development: Stage 1: initial attachment; stage 2: irreversible attachment; stage 3: maturation I; stage 4: maturation II; stage 5: dispersion. Each stage of development in the diagram is paired with a photomicrograph of a developing Pseudomonas aeruginosa biofilm. All photomicrographs are shown at the same scale.

Biofilm infections develop gradually and often do not cause immediate symptoms. They are rarely resolved by host defense mechanisms. Once an infection by a biofilm is established, it is very difficult to eradicate because biofilms tend to be resistant to most of the methods used to control microbial growth, including antibiotics. Biofilms respond poorly or only temporarily to antibiotics. It has been said that they can resist up to 1,000 times the antibiotic concentrations used to kill the same bacteria when they are free-living or planktonic. An antibiotic dose that large would harm the patient; therefore, scientists are working on new ways to eradicate biofilms.

Antibiotics: Are We Facing a Crisis?

Excessive use of antibiotics in animals or as imprudent medical treatments has resulted in the propagation of antibiotic-resistant bacteria.

Learning Objectives

Discuss antibiotic resistance

Key Takeaways

Key Points

  • In antibiotic resistance, antibiotics will kill most of the infecting bacteria leaving behind only the resistant forms, which reproduce, resulting in an increase in the proportion of resistant forms over non-resistant ones.
  • Cold and flu treatments and the medication of livestock are examples of antibiotic misuse responsible for bacterial resistance.
  • Methicillin-resistant Staphylococcus aureus (MRSA) is an example of a dangerous antibiotic-resistant strain of bacteria that can infect sick, as well as healthy people.
  • Due to the growing resistance to antibiotics, scientists believe that we may be returning to a time in which a simple bacterial infection could again detrimentally impact human populations.

Key Terms

  • antibiotic: any substance that can destroy or inhibit the growth of bacteria and similar microorganisms

Antibiotics: Are We Facing a Crisis?

The word antibiotic comes from the Greek word “anti” meaning “against” and “bios” meaning “life.” An antibiotic is a chemical, produced either by microbes or synthetically, that is hostile to the growth of other organisms. Today’s news and other media often address concerns about an antibiotic crisis. Are the antibiotics that easily treated bacterial infections in the past becoming obsolete? Are there new “superbugs”: bacteria that have evolved to become more resistant to our arsenal of antibiotics? Is this the beginning of the end of antibiotics? All these questions challenge the healthcare community.

One of the main causes of resistant bacteria is the abuse of antibiotics. The imprudent and excessive use of antibiotics has resulted in the natural selection of resistant forms of bacteria. The antibiotic kills most of the infecting bacteria; therefore, only the resistant forms remain. These resistant forms reproduce, resulting in an increase in the proportion of resistant forms over non-resistant ones. Another major misuse of antibiotics is in patients with colds or the flu, for which antibiotics are useless. There is also the excessive use of antibiotics in livestock along with the routine use of antibiotics in animal feed, both of which promote bacterial resistance. In the United States, 70 percent of the antibiotics produced are fed to animals. Because they are given to livestock in low doses, the probability of resistance developing is maximized. These resistant bacteria are readily transferred to humans.

One of the Superbugs: MRSA

The imprudent use of antibiotics has paved the way for bacteria to expand populations of resistant forms. For example, Staphylococcus aureus, often called “staph,” is a common bacterium that can live in the human body and is usually easily treated with antibiotics. A very dangerous strain, however, methicillin-resistant Staphylococcus aureus (MRSA) has made the news over the past few years. This strain is resistant to many commonly-used antibiotics, including methicillin, amoxicillin, penicillin, and oxacillin. MRSA can cause infections of the skin, but it can also infect the bloodstream, lungs, urinary tract, or sites of injury. While MRSA infections are common among people in healthcare facilities, they have also appeared in healthy people who have not been hospitalized, but who live or work in tight populations (like military personnel and prisoners). Researchers have expressed concern about the way this latter source of MRSA targets a much younger population than those residing in care facilities. The Journal of the American Medical Association (JAMA) reported that, among MRSA-afflicted persons in healthcare facilities, the average age is 68, whereas people with “community-associated MRSA” (CA-MRSA) have an average age of 23. Reference: Naimi, TS, LeDell, KH, Como-Sabetti, K, et al. Comparison of community- and health care-associated methicillin-resistant Staphylococcus aureus infection. JAMA 290 (2003): 2976–84, doi: 10.1001/jama.290.22.2976.

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MRSA, a superbug: This scanning electron micrograph shows methicillin-resistant Staphylococcus aureus bacteria, commonly known as MRSA. S. aureus is not always pathogenic, but can cause diseases such as food poisoning and skin and respiratory infections.

In summary, the medical community is facing an antibiotic crisis. Some scientists believe that after years of being protected from bacterial infections by antibiotics, we may be returning to a time in which a simple bacterial infection could again devastate the human population. Researchers are developing new antibiotics, but it takes many years of research and clinical trials, plus financial investments in the millions of dollars, to generate an effective and approved drug.

Bacterial Foodborne Diseases

Foodborne diseases can be associated with bacteria-caused illnesses in both animal and plant-based food sources.

Learning Objectives

Give examples of bacterial foodborne diseases in humans

Key Takeaways

Key Points

  • Food and food-processing equipment are usually colonized by biofilms.
  • A foodborne disease is an illness resulting from the consumption of pathogenic bacteria, viruses, or other parasites that contaminate animal or plant-based food.
  • Proper sterilization techniques and canning procedures have reduced the incidence of botulism.
  • E. coli outbreaks have become more common as new strains continue to evolve.

Key Terms

  • serotype: a group of microorganisms characterized by a specific set of antigens
  • botulism: poisoning caused by the toxin from Clostridium botulinum, a type of anaerobic bacteria that grows in improperly-prepared food

Foodborne Diseases

Prokaryotes are everywhere. They readily colonize the surface of any type of material. Food is not an exception. Most of the time, prokaryotes colonize food and food-processing equipment in the form of a biofilm. Outbreaks of bacterial infection related to food consumption are common. A foodborne disease (colloquially called “food poisoning”) is an illness resulting from the consumption of pathogenic bacteria, viruses, or other parasites that contaminate food. Although the United States has one of the safest food supplies in the world, the U.S. Centers for Disease Control and Prevention (CDC) has reported that “76 million people get sick, more than 300,000 are hospitalized, and 5,000 Americans die each year from foodborne illness.”

The characteristics of foodborne illnesses have changed over time. In the past, sporadic cases of botulism, the potentially fatal disease produced by a toxin from the anaerobic bacterium Clostridium botulinum, were relatively common. Some of the sources for this bacterium were non-acidic canned foods, homemade pickles, and processed meat and sausages. The can, jar, or package created a suitable anaerobic environment where Clostridium could grow. However, proper sterilization and canning procedures have reduced the incidence of this disease.

While people may tend to think of foodborne illnesses as associated with animal-based foods, most cases are now linked to produce. There have been serious, produce-related outbreaks associated with raw spinach in the United States and with vegetable sprouts in Germany. These types of outbreaks have become more common. The raw spinach outbreak in 2006 was produced by the bacterium E. coli serotype O157:H7. A serotype is a strain of bacteria that carries a set of similar antigens on its cell surface. There are often many different serotypes of a bacterial species. Most E. coli are not particularly dangerous to humans, but serotype O157:H7 can cause bloody diarrhea and is potentially fatal.

All types of food can potentially be contaminated with bacteria. Recent outbreaks of Salmonella reported by the CDC occurred in foods as diverse as peanut butter, alfalfa sprouts, and eggs. A deadly outbreak in Germany in 2010 was caused by E. coli contamination of vegetable sprouts. The strain that caused the outbreak was found to be a new serotype not previously involved in other outbreaks, which indicates that E. coli is continuously evolving.

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Bacterial illnesses from food: (a) Vegetable sprouts grown at an organic farm were the cause of an (b) E. coli outbreak that killed 32 people and sickened 3,800 in Germany in 2011. The strain responsible, E. coli O104:H4, produces Shiga toxin, a substance that inhibits protein synthesis in the host cell. The toxin (c) destroys red blood cells, resulting in bloody diarrhea. Deformed red blood cells clog the capillaries of the kidney, which can lead to kidney failure, as happened to 845 patients in the 2011 outbreak. Kidney failure is usually reversible, but some patients experience kidney problems years later.