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A. Introduction

The communicability of organisms responsible for transmitting diseases in humans depends on a variety of factors which will be explained throughout this publication. There are many terms that are utilized in understanding this aspect of microbiology. It is important that some of these terms are explained in the beginning. For example:

• allergen - a chemical substance that elicits a hypersensitive response;

• anaphylactic shock - an immediate sometimes fatal reaction that follow in a human or animal host by contact with the offending allergen through injection, ingestion or inhalation;

• autogenous infection - infection derived from the patients own flora; similar to endogenous infection;

• carrier - an individual who is colonized with a disease producing organism but shows no overt symptoms;

• colonization - implies the presence of microorganisms either in or outside the host without causing any type of response in the host;

• communicable - capable of being transmitted especially when referring to diseases;

• contamination - presence of microorganisms that are transiently present on either a body surface or inaminate objects such as linens, water, food, etc.

• dissemination - shedding of organisms into the environment host - the organism in or on which a parasite lives deriving its nutrients or energy from it (host);

• endogenous infection - infection caused by an organism growing within a host’s body

• epidemiology - study of the relationship of diseases- frequency and distribution;

• flora - population of microorganisms inhabiting the internal and external surfaces of healthy humans or animals;

• fomites - inaminate objects such as linens;

• horizontal transmission - infections from one person to another;

• immune response - a series of complicated reactions that may be beneficial in protecting the host from disease such as through vaccination or harmful causing injury to tissues or death such as in anaphylactic shock;

• incubation period - the time interval that occurs during exposure to a disease producing organism and the initial stages of microbial disease; the interval of time may be from a few hours to many years.

• infection - the replication of organisms in tissues of a human or animal host which may or may not develop into a clinical disease;

• nosocomial infections - infections that are developed within a hospital or are produced by organisms acquired during hospitalization;

• opportunistic infections - infections normally caused by microorganisms that are generally harmless but finds an opportunity to cause disease in an individual whose resistance is lowered by some type of underlying disease or Immunodeficiency;

• parasite - an organism that lives on or in another and obtains its nutrients from it;

• parenteral route - introduction of substances by intravenous, subcutaneous, intramuscular or intramedullary injection.

• pathogenic organisms - disease producing organisms;

• reservoir of infection - living or nonliving material in or on which an infectious agent multiplies and or develops and is dependent for its survival in nature;

• septicemia - disease caused by the spread of organisms and their enzymes and toxins via circulating blood;

• source of infection - location from which an infection is acquired;

• standard precautions - term developed by the Centers for Disease Control and Prevention on handling all body fluids as infectious regardless of source;

• vector - an agent or a living organism such as an insect that can transmit disease producing organisms from one host to another;

• vertical transmission - infections spread from mother to fetus.

 

B. Modes of Transmission

The communicability of infections is dependent on the accessibility of the reservoir and routes that are available for transmission. There are a variety of ways organisms can enter a human host. For example, viruses like influenza commonly enter by the respiratory route, organisms like Shigella dysenteriae enter by the gastrointestinal route, Staphylococcus aureus by breaks in skin or mucous membranes, malaria by biological vectors such as the mosquito and hepatitis B by the parenteral route.

Some organisms may have more than one way of entering a host. Organisms such as Mycobacterium tuberculosis primarily enter by the airborne route by creating small droplet nuclei (particles the size of <5 micrometers) that can enter very easily into the lung. The mumps virus can enter by way of droplet spread or direct contact with saliva of an infected individual. Hepatitis B virus can enter by the parenteral, sexual or vertical route. Salmonella typhi is spread by contaminated food through human sewage such as oysters, or by fecally contaminated water which is in both cases generally referred to as common-vehicle transmission. Malaria is transferred by the bite of an infective female (Anopheles) mosquito which is referred to as vector borne transmission. It is important to note here that there are at least two ways vectors can transmit disease to humans: biologically, where the vector is used as part of the life cycle in reproduction such as in malaria, and mechanical where there is physical transference of organism by an insect such as flies from fecal contamination obtained from landfills or sewer drainage.

Knowing how organisms are transferred to the human host is helpful in determining how nosocomial infections occur. This information can lead to the source of the problem and may give rise to specific control measures. For example, when one observes that in a particular ward in a nursing home setting has a very alarming number of urinary tract infections caused by enteric organisms such as Escherichia coli or Proteus mirabilis or Enterococcus faecalis, one would expect poor hygiene or lack of perineal cleansing among the residents. The staff development person observing this data would stress to the nursing staff the need for better hygienic behavior among the residents

Table 1 is a compilation of known endogenous flora that can be normally isolated from humans without expression of any clinical signs. It is so important to realize that the human host’s own flora could cause infections especially when the patient is compromised immunologically such as in cancer or AIDS. Remember that the human host can become colonized with organisms that are known to cause disease (pathogenic). It is not unusual to isolate these organisms from specimens that are submitted for microbiological analysis. Sometimes, unfortunately, patients are treated for colonization rather than infection. When this occurs the health-care facilities are creating "super bugs" which makes the use of antibiotics less effective.

Table 1 - Endogenous flora as potential pathogens¹

Organism	Normal location	Frequency	Mode of transmission	Common infections
Gram positive
1. Staphylococcus aureus	skin, hair, axilla, perianal, anterior nares, mouth	common	contact, rarely airborne or through fomites	skin lesions, abscesses
2. Staphylococcus epidermidis	skin, hair, nasopharnyx, mouth, vagina	very common	contact	skin lesions, prosthetic contamination
3. Streptococcus pyogenes (Group A)	orapharynx, perianal, anal	uncommon (5 - 10%)	contact, rarely airborne or through fomites	pharyngitis
4. Streptococcus agalactiae (Group B)	adult vagina, genitalia, colon	uncommon (10 - 30% of pregnant females	contact	neonatal meningitis
5. Enterococcus faecalis	colon	common	contact2	urinary tract infection, endocarditis
6. Streptococcus pneumoniae	oropharynx	uncommon (<25%)	droplet spread, direct oral contact	pneumonia, otitis media
7. Streptococcus sp.	oropharynx, skin	very common	contact	endocarditis
8. Corynebacterium sp.	skin, mouth, vagina	very common	contact	endocarditis
Yeasts
1. Candida albicans	mouth, skin, vagina, colon	common	contact	mucocutaneous
2. Candida sp.	mouth, skin, vagina	common	contact	mucocutaneous
Gram negative
1. Escherichia coli	colon, perineum	very common	contact	urinary tract infections, wounds
2. Klebsiella pneumoniae	colon, perineum	common	contact	urinary tract, pneumonia
3. Proteus mirabilis	colon, perineum	common	contact	urinary tract infections, wounds
4. Serratia sp.	colon, perineum	uncommon	contact, environmental	urinary tract infections, wounds, pneumonia
5. Pseudomonas aeruginosa	colon	uncommon (less than 10% of population are colonized)	contact, environmental3	urinary tract infections, wounds and pneumonia (especially in cystic fibrosis patients
6. Hemophilus influenzae	oropharynx	common (as high as 50% of the population - non-type b strains)	contact	meningitis, pneumonia
7. Neisseria meningitis	oro- or nasopharynx	uncommon (5 - 10%)	close contact such as mouth to mouth	meningitis
Anaerobic bacteria
1. Bacteriodes sp.	colon, vagina	very common	contact	septicemia
2. Clostridium sp.	colon, vagina	common	contact	septicemia
3. Fusobacterium sp.	colon, upper respiratory	common	contact	septicemia
Molds	ubiquitous	disease due to compromising conditions	airborne	pneumonia
Viruses	>400 are classified; role as endogenous flora is undetermined		airborne	variety

Escherichia coli is normally found in the colon (See Table 1). This organism is the most common cause of urinary tract infections especially in the elderly.

 

The three most common isolates of urinary tract infections are Escherichia coli, Proteus mirabilis and Enterococcus sp. If a nurse examines this report which was either compiled  by the laboratory or the person doing the surveillance, would quickly see that there are increasing numbers of these isolates over time. This indicates that there is a major problem with urinary tract infections - they are continually rising over time. Since these organisms are normally found in the intestine (See Table 1), the probability that hygiene of patients/residents is suspect. Knowing the source of organisms can contribute to reduction, prevention and/or control of infections.

As mentioned above there are a variety of ways microorganisms can enter a human host: contact, airborne, common vehicle and vector-borne. Contact transmission is divided into two sub-groups: direct contact which involves direct body surface-to-body surface contact which allows the physical transfer of organisms from one person to another and indirect contact which involves contact of a susceptible host with a contaminated object (usually fomites) such as contaminated instruments, dressings or grossly contaminated linens or even gloves that are not changed from one site to another from the same patient ( e.g., perineal cleansing and G-tube examination).

Airborne transmission occurs when organisms are spread by droplet nuclei (< 5 micrometers or smaller in diameter) of evaporated droplets containing microorganisms that will remain suspended in the air for prolonged periods of time especially when there is improper ventilation.

Common vehicle transmission applies when microorganisms are transmitted by contaminated food, water, medications, devices and equipment. Foodborne illnesses are becoming more prevalent in the United States especially with Escherichia coli 0157:H7

 

Vector-borne transmission occurs when vectors such as mosquitoes, or ticks infect humans directly.

 

Sometimes rats or other vermin act as reservoirs for organisms that infect humans or animals such as the Hantaviruses. Fortunately, this method of transmission is rare in the United States.

Table 2 list the common pathogenic organisms, sites of infection, the typical disease(s) they produce, incubation periods, modes of transmission and precautionary methods for prevention. The Centers for Disease Control and Prevention proposed in 1996 new terminology for transmission-based precautions designed for patients/residents known or suspected to be infected with highly transmissible or epidemiologically important pathogens for which additional precautions beyond Standard Precautions (formerly Universal Precautions) are needed to interrupt transmission in a healthcare setting regardless of type.

There are three types of transmission-based precautions:

• airborne precautions

• droplet precautions

• contact precautions

Airborne precautions are designed to reduce the risk of airborne transmission of infectious agents such as Mycobacterium tuberculosis which creates small particles of <5 micrometers or less. This microorganism can be easily carried by air currents and may become inhaled. Special ventilation systems must be employed to prevent this from happening (special rooms with air-exchanges and negative pressures).

Droplet precautions involves contact of the conjunctivae, or mucous membranes of the nose or mouth of a particle usually > 5 micrometers in diameter. Droplets are generated from the source person during coughing, sneezing, or suctioning. Transmission by this route requires close contact (3 feet or less). Since these droplets are large, they do not remain suspended in the air for long periods of time.

Contact precautions is designed to reduce the spread of organisms by direct or indirect contact. Direct contact transmission involves skin to skin contact such as turning a patient/resident. Sometimes the environment can contribute to such contamination (Clostridium difficile or Enterococcus faecalis). If a particular patient/resident is incontinent and contamination of the environment is likely, then a private room is generally used. Often patients/residents are placed in contact isolation if they are either infected or colonized with an epidemiologically important organism such as Methicillin-resistant Staphylococcus aureus or vancomycin-resistant Enterococcus sp. Private rooms are commonly used for contact isolation. Ventilation systems are not required in such rooms.

Table 2 - Common pathogenic organisms and modes of transmission¹

Organism	Site(s)	Disease(s)	Incubation period	Mode of transmission/ precautions
Gram positive
1.Corynebacterium diphtheriae	throat	diphtheria	2-5 days	contact/droplet spread
2. Streptococcus pneumoniae	lower respiratory tract	pneumonia, meningitis	1-3 days	contact/droplet spread
3. Listeria monocytogenes	meningoencephalitis	listeriosis	3-70 days	ingestion of raw or contaminated milk, soft cheeses, contaminated vegetables/standard
4. Bacillus anthracis	lower respiratory tract, skin lesions	anthrax	few hours - 7 days	contact with tissues of infected animal or products (i.e., hides, wool - occupational); ingestion of uncooked meats contaminated with organism/standard
5. Staphylococcus aureus	skin , osteomyelitis, blood, heart	boils, furuncles, abscesses, impetigo, osteomyelitis, sepsis, toxic shock syndrome	4-10 days	contact, autoinfection/ major lesions or drug resistant - contact
6. Streptococcus pyogenes	throat, skin, blood, middle ear	pharyngitis, septicemia, erysipelas, rheumatic fever, scarlet fever, otitis media, food borne illness	short, usually 1 - 3 days	direct contact/droplet spread
Gram negative
1. Bordetella pertussis	oropharynx	whooping cough	6-20 days	direct contact with discharges from respiratory mucous membranes of infected persons by the airborne route/droplet spread
2. Escherichia coli (O157:H7)	large intestine	hemorrhagic colitis	3-8 days	ingestion of undercooked hamburger/standard
3. Legionella sp.	lower respiratory tract	legionellosis	2-10 days	airborne (environmental, non-communicable)
4. Neisseria gonorrhoeae	genitourinary tract, eye	gonorrhea, pelvic inflammatory disease, septicemia, pharyngitis	2-7 days	sexual (standard precautions)
5. Neisseria meningitidis	meninges	meningitis	2-10 days	direct contact/ droplet spread
6. Salmonella sp.	small intestine	gastroenteritis	6-72 hours	ingestion/contact2
7. Salmonella typhi	small intestine	typhoid fever	1-3 weeks	ingestion/ standard2
8. Shigella sp.	large intestine	shigellosis (enteritis); bacterial dysentery	1-3 days	ingestion/ standard2
9. Yersinia sp.	large intestine	enterocolitis, acute mesenteric lymphadenitis	3-7 days	ingestion/ standard2
10. Vibrio sp.	large intestine	cholera, enteritis	2-3 days	ingestion/ standard2
Anaerobes
Gram positive anaerobes
1. Actinomycetes sp.	jaw, thorax, abdomen	chronic abscesses, draining sinuses	irregular, probably many years	endogenous
2. Clostridium botulinum	acute bilateral cranial nerve impairment	botulism	12-36 hours	ingestion of pre-formed toxin (non-communicable)
3. Clostridium difficile	large intestine	pseudomembranous colitis		environmental/contact
4. Clostridium perfringens	skin lesion, large intestine	gas gangrene, food poisoning	food poisoning 6-24 hr; gas gangrene 1-4 days	skin lesions, ingestion (non-communicable)
5. Clostridium tetani	nerve - muscular contractions	tetanus	3-21 days	lesions (non-communicable)
Gram negative anaerobes
1. Bacteriodes sp.	large intestine	peritonitis, endometritis, abscesses, septicemia	unknown	endogenous
Acid-fast
1. Mycobacterium tuberculosis	lower respiratory tract, laryngeal, meningeal	tuberculosis	4-12 weeks	airborne (small particles <5um in diameter)/airborne
2. Mycobacterium avium complex	lower respiratory tract, lymph nodes	pulmonary, lymphadenitis	unknown	ingestion, skin lesions (non-communicable)
Yeasts
1. Candida albicans	mucocutaneous, skin	oral thrush, intertrigo, vulvovaginitis, paronychia	variable - 2-5 days in infants