14.5: Bacteria and human disease

Bacterial Diseases in Humans

Devastating pathogen-borne diseases and plagues, both viral and bacterial in nature, have affected and continue to affect humans. It is worth noting that all pathogenic prokaryotes are Bacteria; there are no known pathogenic Archaea in humans or any other organism. Pathogenic organisms evolved alongside humans. In the past, the true cause of these diseases was not understood, and some cultures thought that diseases were a spiritual punishment or were mistaken about material causes. Over time, people came to realize that staying apart from afflicted persons, improving sanitation, and properly disposing of the corpses and personal belongings of victims of illness reduced their own chances of getting sick.

Historical Perspective

There are records of infectious diseases as far back as 3,000 B.C. A number of significant pandemics caused by Bacteria have been documented over several hundred years. Some of the largest pandemics led to the decline of cities and cultures. Many were zoonoses that appeared with the domestication of animals, as in the case of tuberculosis. A zoonosis is a disease that infects animals but can be transmitted from animals to humans.

Infectious diseases remain among the leading causes of death worldwide. Their impact is less significant in many developed countries, but they are important determiners of mortality in developing countries. The development of antibiotics did much to lessen the mortality rates from bacterial infections, but access to antibiotics is not universal, and the overuse of antibiotics has led to the development of resistant strains of bacteria. Public sanitation efforts that dispose of sewage and provide clean drinking water have done as much or more than medical advances to prevent deaths caused by bacterial infections.

In 430 B.C., the plague of Athens killed one-quarter of the Athenian troops that were fighting in the Great Peloponnesian War. The disease killed a quarter of the population of Athens in over 4 years and weakened Athens’ dominance and power. The source of the plague may have been identified recently when researchers from the University of Athens were able to analyze DNA from teeth recovered from a mass grave. The scientists identified nucleotide sequences from a pathogenic bacterium that causes typhoid fever.¹

From 541 to 750 A.D., an outbreak called the plague of Justinian (likely a bubonic plague) eliminated, by some estimates, one-quarter to one-half of the human population. The population in Europe declined by 50 percent during this outbreak. Bubonic plague would decimate Europe more than once.

One of the most devastating pandemics was the Black Death (1346 to 1361), which is believed to have been another outbreak of bubonic plague caused by the bacterium Yersinia pestis. This bacterium is carried by fleas living on black rats. 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. There are still approximately 1,000 to 3,000 cases of plague globally each year. Although contracting bubonic plague before antibiotics meant almost certain death, the bacterium responds to several types of modern antibiotics, and mortality rates from plague are now very low.

CONCEPT IN ACTION

Watch a video on the modern understanding of the Black Death (bubonic plague) in Europe during the fourteenth century.

Over the centuries, Europeans developed resistance to many infectious diseases. However, European conquerors brought disease-causing bacteria and viruses with them when they reached the Western hemisphere, triggering epidemics that completely devastated populations of Native Americans (who had no natural resistance to many European diseases).

The Antibiotic Crisis

The word antibiotic comes from the Greek anti, meaning “against,” and bios, meaning “life.” An antibiotic is an organism-produced chemical that is hostile to the growth of other organisms. Today’s news and media often address concerns about an antibiotic crisis. Are antibiotics that were used to treat bacterial infections easily treatable 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 of these questions challenge the healthcare community.

One of the main reasons for resistant bacteria is the overuse and incorrect use of antibiotics, such as not completing a full course of prescribed antibiotics. The incorrect use of an antibiotic results 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 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. The antibiotics are not used to prevent disease, but to enhance production of their products.

Staphylococcus aureus, often called “staph,” is a common bacterium that can live in and on the human body, which usually is easily treatable with antibiotics. A very dangerous strain, however, has made the news over the past few years (Figure \(\PageIndex{6}\)). This strain, methicillin-resistant Staphylococcus aureus (MRSA), is resistant to many commonly used antibiotics, including methicillin, amoxicillin, penicillin, and oxacillin. While MRSA infections have been common among people in healthcare facilities, it is appearing more commonly in healthy people who live or work in dense groups (like military personnel and prisoners). The Journal of the American Medical Association reported that, among MRSA-afflicted persons in healthcare facilities, the average age is 68 years, while people with “community-associated MRSA” (CA-MRSA) have an average age of 23 years.²

Figure \(\PageIndex{6}\): This scanning electron micrograph shows methicillin-resistant Staphylococcus aureus bacteria, commonly known as MRSA. (credit: modification of work by Janice Haney Carr, CDC; scale-bar data from Matt Russell)

In summary, society 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 working on developing new antibiotics, but few are in the drug development pipeline, and it takes many years to generate an effective and approved drug.

Foodborne Diseases

Prokaryotes are everywhere: They readily colonize the surface of any type of material, and food is not an exception. 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 food contaminated with pathogenic bacteria, viruses, or other parasites. Although the United States has one of the safest food supplies in the world, the Center 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.”³

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. A 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.

Most cases of foodborne illnesses are now linked to produce contaminated by animal waste. For example, there have been serious, produce-related outbreaks associated with raw spinach in the United States and with vegetable sprouts in Germany (Figure \(\PageIndex{7}\)). The raw spinach outbreak in 2006 was produced by the bacterium E. coli strain O157:H7. Most E. coli strains are not particularly dangerous to humans, (indeed, they live in our large intestine), but O157:H7 is potentially fatal.

Figure \(\PageIndex{7}\): (a) Locally grown vegetable sprouts were the cause of a European E. coli outbreak that killed 31 people and sickened about 3,000 in 2010. (b) Escherichia coli are shown here in a scanning electron micrograph. The strain of E. coli that caused a deadly outbreak in Germany is a new one not involved in any previous E. coli outbreaks. It has acquired several antibiotic resistance genes and specific genetic sequences involved in aggregation ability and virulence. It has recently been sequenced. (credit b: Rocky Mountain Laboratories, NIAID, NIH; scale-bar data from Matt Russell)

All types of food can potentially be contaminated with harmful bacteria of different species. Recent outbreaks of Salmonella reported by the CDC occurred in foods as diverse as peanut butter, alfalfa sprouts, and eggs.

CAREERS IN ACTION: Epidemiologist

Epidemiology is the study of the occurrence, distribution, and determinants of health and disease in a population. It is, therefore, related to public health. An epidemiologist studies the frequency and distribution of diseases within human populations and environments.

Epidemiologists collect data about a particular disease and track its spread to identify the original mode of transmission. They sometimes work in close collaboration with historians to try to understand the way a disease evolved geographically and over time, tracking the natural history of pathogens. They gather information from clinical records, patient interviews, and any other available means. That information is used to develop strategies and design public health policies to reduce the incidence of a disease or to prevent its spread. Epidemiologists also conduct rapid investigations in case of an outbreak to recommend immediate measures to control it.

Epidemiologists typically have a graduate-level education. An epidemiologist often has a bachelor’s degree in some field and a master’s degree in public health (MPH). Many epidemiologists are also physicians (and have an MD) or they have a PhD in an associated field, such as biology or epidemiology.

Footnotes

1. 1 Papagrigorakis M. J., Synodinos P. N., Yapijakis C, “Ancient typhoid epidemic reveals possible ancestral strain of Salmonella enterica serovar Typhi, Infect Genet Evol 7 (2007): 126-7.
2. 2 Naimi, T. S., LeDell, K. H., Como-Sabetti, K., et al., “Comparison of community- and health care-associated methicillin-resistant Staphylococcus aureus infection,” JAMA 290 (2003): 2976-2984, doi: 10.1001/jama.290.22.2976.
3. 3 http://www.cdc.gov/ecoli/2006/september, Centers for Disease Control and Prevention, “Multi-state outbreak of E. coli O157:H7 infections from spinach,” September-October (2006).
4. 4 http://www.cbd.int/convention/articl...cles/?a=cbd-02, United Nations Convention on Biological Diversity, “Article 2: Use of Terms.”

Glossary

anaerobic

refers to organisms that grow without oxygen

anoxic

without oxygen

biofilm

a microbial community that is held together by a gummy-textured matrix

bioremediation

the use of microbial metabolism to remove pollutants

Black Death

a devastating pandemic that is believed to have been an outbreak of bubonic plague caused by the bacterium Yersinia pestis

botulism

a disease produce by the toxin of the anaerobic bacterium Clostridium botulinum

capsule

an external structure that enables a prokaryote to attach to surfaces and protects it from dehydration

commensalism

a symbiotic relationship in which one member benefits while the other member is not affected

conjugation

the process by which prokaryotes move DNA from one individual to another using a pilus

cyanobacteria

bacteria that evolved from early phototrophs and oxygenated the atmosphere; also known as blue-green algae

epidemic

a disease that occurs in an unusually high number of individuals in a population at the same time

extremophile

an organism that grows under extreme or harsh conditions

foodborne disease

any illness resulting from the consumption of contaminated food, or of the pathogenic bacteria, viruses, or other parasites that contaminate food

Gram-negative

describes a bacterium whose cell wall contains little peptidoglycan but has an outer membrane

Gram-positive

describes a bacterium that contains mainly peptidoglycan in its cell walls

hydrothermal vent

a fissure in Earth’s surface that releases geothermally heated water

microbial mat

a multi-layered sheet of prokaryotes that may include bacteria and archaea

MRSA

(methicillin-resistant Staphylococcus aureus) a very dangerous Staphylococcus aureus strain resistant to antibiotics

pandemic

a widespread, usually worldwide, epidemic disease

pathogen

an organism, or infectious agent, that causes a disease

peptidoglycan

a material composed of polysaccharide chains cross-linked to unusual peptides

phototroph

an organism that uses energy from sunlight

pseudopeptidoglycan

a component of some cell walls of Archaea

stromatolite

a layered sedimentary structure formed by precipitation of minerals by prokaryotes in microbial mats

transduction

the process by which a bacteriophage moves DNA from one prokaryote to another

transformation

a mechanism of genetic change in prokaryotes in which DNA present in the environment is taken into the cell and incorporated into the genome

Contributors and Attributions

• Samantha Fowler (Clayton State University), Rebecca Roush (Sandhills Community College), James Wise (Hampton University). Original content by OpenStax (CC BY 4.0; Access for free at https://cnx.org/contents/b3c1e1d2-83...4-e119a8aafbdd).