Human Bacterial Diseases

Identify the most common bacterial diseases in humans

People have been afflicted by pathogen-borne diseases and plagues, both viral and bacterial in nature, since the beginning of human history. The cause of these diseases was not understood at the time, and some people believed that diseases were punishments for sins. Over time, people realized that staying away from afflicted persons, and disposing of corpses and personal belongings of the sick, reduced their own chances of getting sick.

Epidemiologists study how diseases spread throughout a community. In epidemics, an unusually high number of people in a population suffer from a disease at the same time. In general, pandemics are widespread and occur worldwide. An endemic disease is one that is always present, usually at low incidence, in a population.

"LEARNING OBJECTIVES

Identify historical plagues and epidemics caused by bacterial diseases.

Recognize common foodborne illnesses.

Explain how antibiotic overuse may be causing "superbugs."

Bacterial Diseases Have A Long History

It is known that infectious diseases have existed since 3000 B.C. There have been a number of significant pandemics associated with bacteria over the last several hundred years. Some of the most notable pandemics led to the decline of entire nations.

In A disease spreads when the pathogen that causes it is passed from person to person. For a pathogen to cause disease, it must be able to reproduce within a host's body and harm the host in some way. Despite advances in medical research and treatment in recent decades, infectious diseases remain among the leading causes of death worldwide..

The Plague of Athens

micrograph depicts pink rod-shaped bacteria.

Figure 1. Salmonella enterica serovar Typhi, a cause of Typhoid fever. 

In 430 B.C., Plague of Athens killed a quarter of the Athenian troops fighting in the great Peloponnesian War. The plague affected people living in overcrowded Athens as well as troops aboard ships returning to Athens. The source of the plague was discovered recently when researchers from the University of Pittsburgh identified some of the bacteria that caused the plague. Athens were able to use DNA from teeth recovered from a mass grave. 

Typhoid fever is caused by Salmonella enterica serovar Typhi, a Gram-negative, rod-shaped gamma protozoan. Typhoid fever, which is spread through feces, leads to intestinal hemorrhage, high fever, delirium, and dehydration. Between 16 and 33 million cases of this re-emerging disease occur every year, resulting in over 200,000 deaths. Carriers of the disease may be asymptomatic. A cook in the early 1900s, named Mary Mallon, unknowingly spread the disease to over fifty people, three of whom died. Salmonella causes food poisoning..

Bubonic Plagues

In the years 541 to 750, an outbreak of a bubonic plague (the Plague of Justinian) reduced by one-quarter to one-half the population of the eastern Mediterranean region. The population of Europe dropped by fifty percent during this outbreak. Bubonic plague would strike Europe more than once.

It is believed that the Black Death (1346 to 1361) was another outbreak of bubonic plague caused by the bacterium Yersinia pestis. The disease is believed to have spread from China along the Silk Road, a network of land and sea trade routes, to the Mediterranean region and Europe, carried by black rats carrying rat fleas on their bodies. The Black Death reduced the world's population from an estimated 450 million to about 350 to 375 million. The bubonic plague struck London again in the mid-1600s (Figure 2). Plague cases are estimated to occur globally each year between 1,000 and 3,000. While the bacterium responds to several types of modern antibiotics, mortality rates from plague are very low nowadays.

Migration of Diseases to New Populations

The Europeans developed genetic immunity to endemic infectious diseases over the centuries, The European conquerors brought diseases with them to the western hemisphere, which led to epidemics that decimated Native American populations, who had no natural resistance to many European diseases. It has been estimated that up to 90 percent of Native Americans perished from infectious diseases after they were conquered by Europeans. 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 (Figure 3) 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; some are of viral origin and some are of bacterial origin. Brucellosis is an example of a prokaryotic zoonosis that is re-emerging in some regions, and necrotizing fasciitis (commonly known as flesh-eating bacteria) has been increasing in virulence for the last 80 years for unknown reasons.

There are some emerging diseases that aren't actually new, but rather diseases that were catastrophic in the past (Figure 4). Many diseases have been eradicated and then become dormant, only to return, sometimes more virulent than before, as was the case with bubonic plague. Others, There are certain diseases whose worldwide re-emergence should be monitored, including two viral diseases (dengue fever and yellow fever), as well as three bacterial diseases (diphtheria, cholera, and bubonic plague). The war against infectious diseases has no foreseeable end..

Biofilms and Foodborne Diseases

Prokaryotes are everywhere: they readily colonize the surface of any material, and food is no exception. They usually grow as a biofilm on food and food-processing equipment.

Biofilms

Remember that biofilms are communities of microbes that are very difficult to destroy. They are responsible for 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.. In healthcare environments, biofilms grow on hemodialysis machines, mechanical ventilators, shunts, and other medical equipment. In fact, 65 percent of all nosocomial infections occur as a result of biofilms. Biofilms can also cause food-borne illnesses because they colonize the surface of vegetable leaves and meat, as well as equipment used to process food..

Infections caused by biofilms can develop gradually; symptoms do not always appear immediately. Generally, host defense mechanisms do not resolve infections caused by biofilms, because biofilms tend to be resistant to most of the methods used to control microbial growth, including antibiotics. Antibiotics do not work well against biofilms; these bacteria are able to resist up to 1,000 times the antibiotic concentrations used to kill them when they are free-living or planktonic. Such a large dose of antibiotic would harm the patient, so scientists are exploring new methods to destroy biofilms.

Foodborne Diseases

Bacterial infections resulting from food consumption are common. Foodborne diseases (also called "food poisoning") are illnesses caused by bacteria, viruses, or parasites present in food. While the U.S. food supply is one of the safest in the world, the U.S. Centers for Disease Control and Prevention report outbreaks of foodborne diseases every year. 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, it was relatively common to hear about sporadic cases of botulism, the potentially fatal disease produced by a toxin from the anaerobic bacterium Clostridium botulinum. Some of the most common 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. Proper sterilization and canning procedures have reduced the incidence of this disease.

People tend to think that foodborne illnesses are linked to animal-based foods, but most cases are now related to produce. Raw spinach in the United States has been associated with serious outbreaks, as have vegetable sprouts in Germany. These types of outbreaks are becoming more common. The raw spinach outbreak in 2006 was produced by the bacterium E. coli serotype O157:H7. There are typically many different serotypes of a bacterial species, and most E. coli are not particularly dangerous to humans, but serotype O157:H7 can cause bloody diarrhea and may be fatal.

CDC reports that Salmonella outbreaks have recently occurred in foods as diverse as peanut butter, alfalfa sprouts, and eggs. An E. coli outbreak in Germany in 2010 was caused by contaminated vegetable sprouts (Figure 5). The strain that caused the outbreak was a new serotype not previously involved in other outbreaks, indicating that E. coli is always evolving.

"EPIDEMIOLOGIST

In epidemiology, health and disease are studied by looking at their occurrence, distribution, and causes within a population. It is, therefore, part of public health. An epidemiologist studies the frequency and distribution of diseases within human populations and environments.

To identify the original mode of transmission of a disease, epidemiologists collect data about its spread. Often, researchers work closely with historians to understand how diseases evolve geographically and over time, tracking the evolution of pathogens. Clinical records, interviews with patients, surveillance, and other methods are used to gather information. A disease's incidence or spread can be reduced through the use of public health policies, such as vaccinations (Figure 6). Additionally, epidemiologists conduct rapid investigations in the event of an outbreak to recommend immediate measures to stop it.

An epidemiologist has a bachelor’s degree, plus a master’s degree in public health (MPH). Many epidemiologists are also physicians (have an M.D.), or they have a Ph.D. in an associated field, such as biology or microbiology."

Antibiotic Resistance

The word antibiotic comes from the Greek 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 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, and 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 because these illnesses are caused by viruses, not bacteria. Another problem is the excessive use of antibiotics in livestock. The routine use of antibiotics in animal feed promotes bacterial resistance as well. In the United States, 70 percent of the antibiotics produced are fed to animals. These antibiotics are given to livestock in low doses, which maximize the probability of resistance developing, and these resistant bacteria are readily transferred to humans.

Drug Resistance

Antimicrobial resistance is not a new phenomenon. In nature, microbes are constantly evolving in order to overcome the antimicrobial compounds produced by other microorganisms. Human development of antimicrobial drugs and their widespread clinical use has simply provided another selective pressure that promotes further evolution. Several important factors can accelerate the evolution of drug resistance. These include the overuse and misuse of antimicrobials, inappropriate use of antimicrobials, subtherapeutic dosing, and patient noncompliance with the recommended course of treatment.

When a pathogen is repeatedly exposed to an antimicrobial, it can select for chromosomal mutations conferring resistance. These mutations can then be transferred to successive microbial generations and eventually become dominant in the microbial community. Alternatively, many of the genes responsible for drug resistance are found on plasmids or transposons that can be transferred easily between microbes via horizontal gene transfer. Transposons can also move resistance genes between plasmids and chromosomes to further encourage resistance.

How Resistance Happens

All animals carry bacteria in their intestines. Antibiotics kill many bacteria, but resistant bacteria can survive and multiply.

• The meat and other products of food animals can be contaminated by these resistant bacteria after they are slaughtered and processed.
  
• Additionally, these bacteria can enter the environment when an animal poop and spread to produce that is irrigated with contaminated water.

People can acquire antibiotic-resistant bacteria from industrial food animals by several direct routes:

• handling contaminated meat improperly or eating it uncooked.
  
• People who come into contact with infected farm workers or meat processors, or perhaps their families, doctors, and others.
• Drinking contaminated water or eating contaminated crops.
• Air that is vented from concentrated animal housing or that is released during animal transportation.

In the face of increasing drug-resistance, doctors are often forced to prescribe second- or third-choice drugs to treat infections when the bacteria that cause them are resistant to the drug of choice and this drug doesn't work. However, these alternative drugs can be less effective, more toxic, and more expensive. Preserving the effectiveness of antibiotics is vital to protecting human and animal health.

"ONE OF THE SUPERBUGS: MRSA

By overusing antibiotics, bacteria have developed resistance to them. Staphylococcus aureus, often referred to as "staph," is a common bacterium that lives in the human body and is usually easily treated with antibiotics. Methicillin-resistant Staphylococcus aureus (MRSA), however, has been making headlines over the past few years (Figure 7). Many commonly used antibiotics are resistant to this strain, including methicillin, amoxicillin, penicillin, and oxacillin. MRSA can cause skin infections, as well as infections of the bloodstream, lungs, urinary tract, and sites of injury. MRSA infections are common in healthcare facilities, but they have also been reported in people who haven't been hospitalized but live or work in tight areas (such as military personnel and prisoners). Researchers have expressed concerns about the way that this latter source of MRSA targets a much younger population than health care facilities. The Journal of the American Medical Association reported that people with MRSA in healthcare facilities have an average age of 68, while people with "community-associated MRSA" (CA-MRSA) have an average age of 23."

"IN SUMMARY: ANTIBIOTIC RESISTANCE

The medical community is experiencing an antibiotic crisis. Scientists believe that after years of being protected from bacterial infections by antibiotics, we may be returning to a time when a simple bacterial infection could again lead to mass mortality. New antibiotics are being developed, but it takes years of research and clinical trials, plus millions of dollars in financial investment, to develop an effective and approved drug."

Check Your Understanding

Test your understanding of the topics covered in the previous section by answering the questions below. This short quiz does not count toward your grade in the class, and you can take it again as many times as you wish.

You can use the quiz to check your understanding of the previous section and determine whether to (1) study it further or (2) move on to the next section.