22.4: Bacterial Diseases in Humans - Biology

22.4: Bacterial Diseases in Humans - Biology

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22.4: Bacterial Diseases in Humans

Bacterial Infection: Symptoms, Cause and Diagnosis

Bacteria are microscopic, single-celled organisms. There are thousands of different kinds of bacteria that live in every conceivable environment all over the world. They live in soil, seawater, and deep within the earth’s crust. Some bacteria have been reported even to live even in radioactive wastes. Some bacteria live in the bodies of people and animals—on the skin and in the airways, mouth, and digestive and genitourinary tracts—often without causing any harm.

Bacterial infections occur when harmful bacteria enter our body or the existing bacteria get out of balance. The bacteria that cause disease are called pathogens. Sometimes bacteria that normally reside harmlessly in the body can also cause disease. Bacteria can cause disease by producing harmful substances like toxins or by invading the tissues.

Bacteria can be classified in several ways:

  • Scientific names: Their scientific name is genus followed by species e.g. example, Clostridium botulinum. Within a species, there may be different types, called strains which differ in genetic makeup and chemical components. Certain drugs and vaccines are effective only against certain strains.
  • Staining: Gram stain is the most commonly used stain for bacterial staining. Some bacteria stains blue. They are called gram-positive. Others stain pink. They are called gram-negative. Gram-positive and gram-negative bacteria stain differently because their cell walls are different. They also cause different types of infections, and different types of antibiotics are effective against them.
  • Shapes: Bacteria may also be classified on the basis of three basic shapes: spheres (cocci), rods (bacilli), and spirals or helixes (spirochetes).
  • Need for oxygen: Bacteria are also classified by whether they need oxygen to live and grow. The bacteria which need oxygen are called aerobes. Whereas the bacteria that have trouble living or growing when oxygen is present are called anaerobes. Some bacteria, called facultative bacteria, can live and grow both in the absence or presence of oxygen.

Most common causes of bacterial infections:

  • Staphylococci—These often harmless bacteria commonly live in and on the body. Still some species can cause disease or infections.
  • Streptococci—These are common bacteria. Some types can cause infections such as strep throat or other respiratory infections, including pneumonia.
  • Haemophilus influenzae—These are also common type of bacteria that can sometimes cause infections. Harmful types can cause diseases that include respiratory infections, ear infections, etc.
  • E Coli—These bacteria commonly live in the GI tract of animals and humans. Some can cause food poisoning if transmitted through improperly cooked food or other food products that have been contaminated.
  • Pylori—These bacteria are a common cause of stomach ulcers.
  • Salmonella—This is another food borne pathogen that causes diarrhea or food poisoning. Some bacterial diseases can be identified clinically. However, most of the bacteria cause a wide range of clinical syndromes. And a single clinical syndrome may result from infection with any one of the many bacteria. Therefore, it is necessary to use microbiological laboratory methods to identify a specific etiologic agent. Diagnostic medical microbiology is the discipline that identifies etiologic agents of disease. The job of the clinical microbiology laboratory is to test specimens from patients for micro-organisms that are (or may be) a cause of illness and to provide information about the in vitro activity of antimicrobial drugs against the micro-organisms identified. Infectious disease with bacterial etiology can be confirmed with the laboratory procedures summarized in the Fig.1.

Considerable efforts have been made for the development of rapid, sensitive and specific assays for the detection of causative organisms. In the last few years many new techniques have entered into the field of early diagnostic process of bacterial infections. Diagnosis of bacterial infections can be classified broadly into the following four types:

Microscopic Diagnosis of Bacterial Infection

  1. Unstained Method:
  • It is commonly used to diagnose bacterial vaginosis. It includes wet preparation and potassium hydroxide (KOH) examination for microscopic examination of samples from the vaginal mucosa and skin surrounding the vaginal opening.
  • Dark-ground illumination examination: Fresh samples can be prepared into slides and studied under dark field illumination. A newer method involves preparing slides from dried fluid smears and staining them with fluorescein for viewing under UV light. This method is replacing dark-field examination because the slides can be transported to professional laboratories.
  1. Stained method: Gram staining, Acid-fast staining

Culture Method

  • Non-radiometric blood culture systems: One of the most important tasks performed by the clinical microbiology laboratory is the detection of bloodstream infections. Rapid bacterial identification and susceptibility testing use fluorescence-based technology. They are widely used because of the lower contamination risks, higher isolation rates and shorter incubation periods. These methods are both rapid and sensitive but still false-positive and false-negative results are high.
  • The rapid evaluation of bacterial growth and antibiotic susceptibility in blood cultures by selected ion flow tube mass spectrometry (SIFT-MS): Selected ion flow tube mass spectrometry measures metabolic gases in the headspaces of blood culture bottles which helps in achieving faster bacterial diagnosis.
  • In many cases, the cause of a bacterial infection is confirmed by isolating and culturing microorganism either in artificial media (either liquid (broth) or on solid (agar)) or in a living host. In some cases, we can take advantage differential media (e.g., eosin methylene blue or MacConkey agar) which are commonly used for the isolation of enteric bacilli. Culture media can also be made selective by addition of compounds such as antimicrobial agents that inhibit the indigenous flora while permitting growth of specific micro-organisms resistant to these inhibitors. For example, Neisseria gonorrhoeae can be isolated with Thayer-Martin medium. Thayer-Martin medium contains vancomycin which inhibits Gram-positive bacteria, colistin which inhibits most of the Gram-negative bacilli, trimethoprim-sulfamethoxazole which inhibits Proteus species and other species that are not inhibited by colistin and anisomycin which inhibits fungi. The pathogenic Neisseria species, N gonorrhoeae and N meningitidis, are ordinarily resistant to the concentrations of these antimicrobial agents in the medium.
  • Presence of bacterial infection can also be defined by the number of bacteria in specimens. For this quantitative cultures have to be performed. For other specimens a semi-quantitative streak method over the agar surface is sufficient. For quantitative cultures (Fig.2), a specific volume of specimen is spread over the agar surface and the number of colonies per milliliter is estimated. For semi-quantitative cultures, an unknown amount of specimen is applied onto the agar and then diluted by being streaked out from the inoculation site with a sterile bacteriologic loop. The growth on the agar is then reported semi-quantitatively as many, moderate, or few (3+, 2+, 1+ respectively), depending on how far colonies appear from the inoculum’s site. An organism that grows in all streaked areas is reported as 3+.

Bacterial cultures are incubated at 35°C to 37°C in an atmosphere which consists of air, air supplemented with carbon dioxide (3-10%), reduced oxygen (micro-aerophilic conditions), or no oxygen (anaerobic conditions), depending upon the requirements of the micro-organism. Bacterial infections samples often contain aerobic, facultative anaerobic and anaerobic bacteria. Therefore, such samples are usually inoculated with a general purpose, differential, and selective media and finally are then incubated under aerobic and anaerobic conditions (Fig.3). The incubation period of cultures varies with the growth characteristics of the micro-organism. Most aerobic and anaerobic bacteria grow overnight, whereas some mycobacterium require 6 to 8 weeks.

Antimicrobial Susceptibility

The term susceptible means that the micro-organism is inhibited by an antimicrobial agent and implies that an infection caused by this micro-organism can be treated with the antimicrobial agent. In addition to microbial detection and isolation, the microbiology laboratory also determines the microbial susceptibility to antimicrobial agents. Many bacteria have unpredictable susceptibilities to antimicrobial agents which can be measured in vitro for the selection of the most appropriate antimicrobial agent. Antimicrobial susceptibility tests are performed by either disk diffusion or a dilution method. In disk diffusion method, a suspension (standardized) of a particular micro-organism is inoculated onto an agar surface. Then paper disks containing various antimicrobial agents are applied on it. After overnight incubation, diameters of inhibition about the disks are measured and the results are reported as indicating susceptibility or resistance of the micro-organism to each antimicrobial agent tested. An alternative method is to dilute on a log2 scale each antimicrobial agent in broth to provide a range of concentrations and to inoculate each tube containing the antimicrobial] agent in broth with a standardized suspension of the micro- organism to be tested. The lowest concentration of antimicrobial agent that inhibits the growth of the micro-organism is the minimal inhibitory concentration (MIC). The MIC and the zone diameter of inhibition are inversely correlated. In other words, the more susceptible the micro-organism is to the antimicrobial agent, the lower the MIC and the larger the zone of inhibition. In Fig.4, two different micro-organisms are tested by both methods (disk diffusion or a dilution method) against the same antibiotic. The MIC of the antibiotic for the susceptible micro-organism is 8p1g/ml. The corresponding disk diffusion test shows a zone of inhibition surrounding the disk. In the second sample, a resistant micro-organism is not inhibited by the highest antibiotic concentration tested (MIC > 16 pg/ml) and there is no zone of inhibition surrounding the disk. The diameter of the zone of inhibition is inversely related to the MIC.


Why study ecology? Perhaps you are interested in learning about the natural world and how living things have adapted to the physical conditions of their environment. Or, perhaps you’re a future physician seeking to understand the connection between human health and ecology.

Humans are a part of the ecological landscape, and human health is one important part of human interaction with our physical and living environment. Lyme disease, for instance, serves as one modern-day example of the connection between our health and the natural world (Figure 35.1). More formally known as Lyme borreliosis, Lyme disease is a bacterial infection that can be transmitted to humans when they are bitten by the deer tick (Ixodes scapularis), which is the primary vector for this disease. However, not all deer ticks carry the bacteria that will cause Lyme disease in humans, and I. scapularis can have other hosts besides deer. In fact, it turns out that the probability of infection depends on the type of host upon which the tick develops: a higher proportion of ticks that live on white-footed mice carry the bacterium than do ticks that live on deer. Knowledge about the environments and population densities in which the host species is abundant would help a physician or an epidemiologist better understand how Lyme disease is transmitted and how its incidence could be reduced.

For example, the mild winter in the northeast during 2010–2011 caused a boom in acorns, which in turn caused an increase in the white-footed mice population. However, the following winter was cooler, leading to fewer acorns, and the subsequent decrease in the mice population means that the ticks will be more likely to seek out humans for their blood meals. You can read more about the relationship between acorns, mice, and Lyme disease at the Science Daily website and you can read more about Lyme disease from the CDC website.

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    Pathways to zoonotic spillover

    Zoonotic spillover, which is the transmission of a pathogen from a vertebrate animal to a human, presents a global public health burden but is a poorly understood phenomenon. Zoonotic spillover requires several factors to align, including the ecological, epidemiological and behavioural determinants of pathogen exposure, and the within-human factors that affect susceptibility to infection. In this Opinion article, we propose a synthetic framework for animal-to-human transmission that integrates the relevant mechanisms. This framework reveals that all zoonotic pathogens must overcome a hierarchical series of barriers to cause spillover infections in humans. Understanding how these barriers are functionally and quantitatively linked, and how they interact in space and time, will substantially improve our ability to predict or prevent spillover events. This work provides a foundation for transdisciplinary investigation of spillover and synthetic theory on zoonotic transmission.

    World Health Organization Estimates of the Global and Regional Disease Burden of 22 Foodborne Bacterial, Protozoal, and Viral Diseases, 2010: A Data Synthesis

    Background: Foodborne diseases are important worldwide, resulting in considerable morbidity and mortality. To our knowledge, we present the first global and regional estimates of the disease burden of the most important foodborne bacterial, protozoal, and viral diseases.

    Methods and findings: We synthesized data on the number of foodborne illnesses, sequelae, deaths, and Disability Adjusted Life Years (DALYs), for all diseases with sufficient data to support global and regional estimates, by age and region. The data sources included varied by pathogen and included systematic reviews, cohort studies, surveillance studies and other burden of disease assessments. We sought relevant data circa 2010, and included sources from 1990-2012. The number of studies per pathogen ranged from as few as 5 studies for bacterial intoxications through to 494 studies for diarrheal pathogens. To estimate mortality for Mycobacterium bovis infections and morbidity and mortality for invasive non-typhoidal Salmonella enterica infections, we excluded cases attributed to HIV infection. We excluded stillbirths in our estimates. We estimate that the 22 diseases included in our study resulted in two billion (95% uncertainty interval [UI] 1.5-2.9 billion) cases, over one million (95% UI 0.89-1.4 million) deaths, and 78.7 million (95% UI 65.0-97.7 million) DALYs in 2010. To estimate the burden due to contaminated food, we then applied proportions of infections that were estimated to be foodborne from a global expert elicitation. Waterborne transmission of disease was not included. We estimate that 29% (95% UI 23-36%) of cases caused by diseases in our study, or 582 million (95% UI 401-922 million), were transmitted by contaminated food, resulting in 25.2 million (95% UI 17.5-37.0 million) DALYs. Norovirus was the leading cause of foodborne illness causing 125 million (95% UI 70-251 million) cases, while Campylobacter spp. caused 96 million (95% UI 52-177 million) foodborne illnesses. Of all foodborne diseases, diarrheal and invasive infections due to non-typhoidal S. enterica infections resulted in the highest burden, causing 4.07 million (95% UI 2.49-6.27 million) DALYs. Regionally, DALYs per 100,000 population were highest in the African region followed by the South East Asian region. Considerable burden of foodborne disease is borne by children less than five years of age. Major limitations of our study include data gaps, particularly in middle- and high-mortality countries, and uncertainty around the proportion of diseases that were foodborne.

    Conclusions: Foodborne diseases result in a large disease burden, particularly in children. Although it is known that diarrheal diseases are a major burden in children, we have demonstrated for the first time the importance of contaminated food as a cause. There is a need to focus food safety interventions on preventing foodborne diseases, particularly in low- and middle-income settings.

    Conflict of interest statement

    The findings and conclusions of this report are those of the authors and do not necessarily represent the official views, decisions or policies of the World Health Organization, US Centers for Disease Control and Prevention, the Department of the Navy, Department of Defense, the US Government or other institutions listed. CFL, AJH, and FJA are employees of the US Government. MDK, REB, MC, BD, DD, AF, TH, KHK, RL, CFL, PRT, AHH, and FJA serve as members of the World Health Organization advisory body—the Foodborne Disease Epidemiology Reference Group—without remuneration. MDK is a member of the Editorial Board for PLOS ONE . PRT is a member of the Editorial Board for PLOS Neglected Tropical Diseases .


    Fig 1. Disability Adjusted Life Years for…

    Fig 1. Disability Adjusted Life Years for each pathogen acquired from contaminated food ranked from…

    Cellular and molecular biology of Neisseria meningitidis colonization and invasive disease

    The human species is the only natural host of Neisseria meningitidis, an important cause of bacterial meningitis globally, and, despite its association with devastating diseases, N. meningitidis is a commensal organism found frequently in the respiratory tract of healthy individuals. To date, antibiotic resistance is relatively uncommon in N. meningitidis isolates but, due to the rapid onset of disease in susceptible hosts, the mortality rate remains approx. 10%. Additionally, patients who survive meningococcal disease often endure numerous debilitating sequelae. N. meningitidis strains are classified primarily into serogroups based on the type of polysaccharide capsule expressed. In total, 13 serogroups have been described however, the majority of disease is caused by strains belonging to one of only five serogroups. Although vaccines have been developed against some of these, a universal meningococcal vaccine remains a challenge due to successful immune evasion strategies of the organism, including mimicry of host structures as well as frequent antigenic variation. N. meningitidis express a range of virulence factors including capsular polysaccharide, lipopolysaccharide and a number of surface-expressed adhesive proteins. Variation of these surface structures is necessary for meningococci to evade killing by host defence mechanisms. Nonetheless, adhesion to host cells and tissues needs to be maintained to enable colonization and ensure bacterial survival in the niche. The aims of the present review are to provide a brief outline of meningococcal carriage, disease and burden to society. With this background, we discuss several bacterial strategies that may enable its survival in the human respiratory tract during colonization and in the blood during infection. We also examine several known meningococcal adhesion mechanisms and conclude with a section on the potential processes that may operate in vivo as meningococci progress from the respiratory niche through the blood to reach the central nervous system.


    Figure 1. Schematic diagram showing structural organization…

    Figure 1. Schematic diagram showing structural organization of N. meningitidis LPS and some important determinants…

    Figure 2. Pili of N. meningitidis

    Figure 2. Pili of N. meningitidis

    (A) Transmission electron micrograph of negatively stained preparations of…

    Figure 3. Structures of the β-barrel outer-membrane…

    Figure 3. Structures of the β-barrel outer-membrane proteins NspA and Opc

    Figure 4. Schematic overview of meningococcal interactions…

    Figure 4. Schematic overview of meningococcal interactions at the epithelial barrier of the nasopharynx and…

    Figure 5. Meningococcal entry into and survival…

    Figure 5. Meningococcal entry into and survival within the vasculature

    Capillaries in close proximity to…

    Figure 6. Meningococcal penetration of the BBB…

    Figure 6. Meningococcal penetration of the BBB and interaction with the meninges leading to meningitis

    Watch the video: Οικολογική αντιμετώπιση ωιδίου (June 2022).


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