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Flashcards in Lectures 3-4 Deck (40):

Bacteria require the following for replication:

• Warmth
• Moisture
• Nutrient
• Oxygen and/or carbon dioxide
• The human body can provide these conditions
and thus bacterial infections are common


In order to cause infections in the human
body, bacteria must:

• Gain access to the skin or other organ systems
• Gain access to food sources in the body
• Escape eradication by immune and other host


• Virulence =

the degree of pathogenicity of an


Mechanisms of virulence:

• Adherence
• Invasion
• Metabolic byproducts (gas, acids)
• Toxins
• Degradative enzymes
• Cytotoxins / endotoxins
• Induction of excessive inflammation
• Stimulation of exaggerated immunologic
• Resistance to antibiotics


Bacteria may induce disease through the following

• Direct tissue destruction
• Release of toxins
• Stimulate immunologic processes that damage the host


Normal flora =

the bacteria that normally colonize a
human host
• Normal flora have positive and negative outcomes on the


 Pathogenic actions of bacteria
• Tissue destruction

• Fermentation and metabolism may produce acids and
gases that are damaging to tissue
• Some bacteria release degradative enzymes that
degrade tissues


 Pathogenic actions of bacteria
• Toxins

• Bacterial constituents that harm tissue or trigger
disruptive physiologic processes
• Some bacteria create pre-formed toxins in foods that
induce disease soon after ingestion (S. aureus, Bacillus
ceresus, Clostridium botulinum)


 Pathogenic actions of bacteria
• Exotoxins

• Produced by both Gm + and Gm – bacteria
• Toxins that are secreted by bacteria that degrade cells
or negatively affect cell function
• Ex) Botulinum toxin
• Superantigens activate T-cells but do not require the
presence of an antigen
− Activate large numbers of T cells, release large amoungs
of interleukins (cytokine storm) causing significant
autoimmune responses


 Pathogenic actions of bacteria
• Endotoxin

• May be found in Gm – cell walls
• Lipopolysaccharide (LPS) serves as an endotoxin and
induces significant inflammatory reactions
− Causes release of cytokines
– Interleukin 1
– Prostaglandins
− Induces septic shock which leads to pulmonary and
cardiovascular collapse (vasodilation, capillary leakage,
alterations of systemic vascular resistance (SVR), etc.
− Hypotension, shock, DIC and death may ensue


When can normal flora cause disease?

Normal flora may induce disease when they
enter body tissues or fluids that they
normally do not occupy


Factors to consider when evaluating
infectious process:

• Bacterial strain involved
• Virulence of the organism
• Size of the inoculum
• Immunologic status of the host
• Location of the infection
• Duration of the infectious process


Sites of entry into the human body:
• Oropharyngeal, upper GI and lower GI infection

• Salmonella
• Shigella
• Yersinia
• Escherichia
• Vibrio
• Campylobacter
• Clostridium
• Bacillus
• Listeria


Sites of entry into the human body:
• Inhalation through URT / LRT

• Mycobacterium
• Nocardia
• Mycoplasma
• Legionella
• Bordetella
• Chlamydia
• Streptococcus
• Hemophilus


 Sites of entry into the human body:
• Skin

• Staphylococcus aureus
• Staphylococcus epidermidis


 Sites of entry into the human body:
• Urogenital

• Neisseria
• Treponema
• Chlamydia


 Sites of entry into the human body:
• Trauma

• Clostridium
• Staphylococcus
• Pseudomonas


 Sites of entry into the human body:
• Arthropod bite

• Rickettsia
• Coxiella
• Francisella
• Borrelia
• Yersinia


Sites of entry into the human body: systemic spread

• Bacteria entering through various portals may
enter the blood
• Once in the blood, bacteria may travel to other
parts of the body via hematogenous spread
• Bacteria may also infect the lymphatic system
and be spread throughout the body


 Colonization

• Once bacteria enter the body, the bacteria may
begin to grow and colonize in that body part
• Bacteria colonize many parts of the human body
• Colonization implies that the bacteria are
sequestering and reproducing in that body part
but are not causing tissue invasion or destruction
• If cellular or physiological damage is being done,
this is the basis of infection


 Adherence

• Bacteria use different methods to adhere to
various body tissues
• Adhesin proteins
• Lipoteichoic acid
• Biofilm
• Fimbriae
• Once anchored, it is more difficult for bacteria to
be “washed away” from the tissue


 Escaping host defenses

• Natural selection favors bacteria that can evade
the defenses of a host organism
• Evasion of defenses allows the bacteria to grow
at an exponential rate and proliferate its DNA
• Through mutation, bacteria have developed
some methods of escaping detection and
eradication by the host:


Mechanisms for escaping host defenses:
• Capsules and slime layers protect bacteria from
immune and phagocytic responses

• Capsules can be slippery which makes it difficult for
phagocytes to adhere to
• Capsules prevent destruction of bacteria even after the
bacteria has been engulfed by a phagocyte
• Biofilms can prevent antibodies and complement from
adhering to the bacteria


 Mechanisms for escaping host defenses:
• Bacteria can evade antibody responses by:

• Antigenic variation
− Some bacteria (N. gonorrhoeae) can vary the structure of
surface antigens to evade an antibody response
• Inactivation of antibodies
− Some bacteria (S. aureus) may produce enzymes like
proteases that prevents an antibody from activating
complement and thus masks the bacteria from being
• Growing intracellularly
− Some bacteria (M. tuberculosis) can grow intracellularly,
even within a macrophage


• Gram stain procedure

• Apply crystal violet solution
• Apply iodine solution (mordant)
• Decolorization with alcohol or acetone
− Gram positive cells with thick cell walls hold purple
iodine color
− Gram negative cells will decolorize and become clear
• Counterstain with safranin (pink)
• Result = Gram-positive organisms will stain purple
and Gram-negative organisms will stain pink


• Not all bacteria can be gram stained
• Exceptions include:

• Bacteria that are old or stationary-phase cultures
• Bacteria exposed to antibiotics may not gram stain
• Mycobacteria
− Have waxy outer shell, may be identified with acid-fast
• Mycoplasma
− Have no peptidoglycan
• Peptidoglycan layer is too thin


Atypical bacteria

• A number of bacteria do not fall into traditional
categorization and are called atypical bacteria
• Considered “incomplete bacteria”
• Mycoplasma
• Ureaplasma
− Smaller than normal bacteria
− Lack a true cell wall
− Pleiomorphic (shape can vary)
• Rickettsia
− Smaller than normal
− May be pleiomorphic
− Obligate intracellular parasites


Metabolic distinctions of bacteria

Bacteria may also be classified on their metabolic
• Aerobic or anaerobic
• Requirement for specific nutrients
• Production of certain metabolic compounds
• Presence of specific enzymes
• Automated procedures have been developed to
rapidly identify various bacteria


bacteria testing- culture

• Using a variety of techniques, bacteria can be
grown in vitro
• Aerobic
• Obligate anaerobes
• Facultative anaerobes
• Using different growth medium and knowledge
of the different morphologic characteristics of
each bacteria, the genus and species of the
bacteria can be identified


There are 2 main methods of determining

1) Microserial broth dilutions
− Involves growth of the microorganism in a number of
different test tubes at a certain inoculum
− Serial dilutions are made with each antibiotic such that
the concentrations in the adjacent tube are 1/2 of the
preceding tube
− The minimum inhibitory concentration is the first tube
that is clear (no bacterial growth)
− This is the concentration of antibiotic required to inhibit
the growth of the bacteria
2) Disk diffusion (Kirby-Bauer)
− This method involves growing bacteria on an agar plate
− Small paper disks impregnated with a standard
antibiotic concentration are placed on the agar
− Multiple different antibiotics are tested
− The antibiotic diffuses from the disk to the surrounding
agar medium
− If the antibiotic is effective in inhibiting microbial
growth, a clear “ring” is visible around the paper disk
(zone of inhibition)
− Susceptibility or resistance is determined by the size of
the zone of inhibition


Specimens used in lab Dx
 Blood

• One of the most important sources of bacteria
• Although bacteremia is not always present,
bacteria in a specific foci may enter the blood
stream and travel hematogenously to other sites
• Even if bacteremia is present, the bacteria may
not be continuously present in blood samples
• Important factors to consider:
• Volume of blood sample
• Number of samples taken for each culture
• Location site of each sample


Specimens used in lab Dx
 Cerebrospinal fluid (CSF)

• Bacterial meningitis is a fulminant disease
associated with high morbidity and mortality
• Antimicrobial treatment is done for bacterial
meningitis (but not fungal or aseptic)
• The common organisms are labile and need to be
processed quickly to avoid false negatives
• S. pneumoniae
• N. meningitidis
• Identification of the causative pathogen is
important in successful treatment
• Samples are obtained via lumbar puncture


Specimens used in lab Dx
 Sterile bodily fluids

• A number of fluids that are normally sterile may
harbor bacteria:
• Peritoneal fluid
• Pleural fluid
• Synovial fluid
• Pericardial fluid
• It is often difficult to get such fluids as the
volumes may be small and require invasive


Specimens used in lab Dx
 URT specimens

• It can be difficult to obtain specimens from these
• Contamination with oropharyngeal flora may
also occur
• Swabs are often used to collect nasopharyngeal
• Direct aspiration of sinuses is possible


Specimens used in lab Dx
 LRT specimens

• Sputum expectoration
• Saline induction
• Direct aspiration
• Bronchoscopy
• The presence of squamous epithelial cells
indicate the specimen is probably contaminated
with mouth flora
• Anaerobic bacteria may cause abscesses which
require needle or surgical aspiration to culture
• Some bacteria require special Cx methods


Specimens used in lab Dx
 Ear and Eye

• Tympanocentesis involves aspiration of fluid
from the middle ear
• Invasive, not typically done in clinical practice
• Complications include:
− Iatrogenic introduction of bacteria into middle ear space
− Scarring
− Meningitis
• Empiric therapy covering common organisms is
common practice
• Specimens obtained from eyes are usually very
• A small number of organisms may be present, making
detection and identification difficult
• Intraocular specimens may be obtained through
direct aspiration of the eye


Specimens used in lab Dx
 Wound tissue

• Specimens can be taken directly from a wound
• It is important to differentiate between surface
microorganisms versus those infecting a wound
• Swabs are often ineffective and can be contaminated
with skin flora
• Abscess aspirates can be taken from the abscess itself
• Drainage of fluid from wounds may help identify


Specimens used in lab Dx
 Urine

• Often utilized in evaluating urinary tract
• Cystitis
• Pyelonephritis
• Urine is normally sterile
• If urine samples are obtained to evaluate the
presence of bacteria, the first part of the urine
stream should not be used
• Pathogenic bacteria may colonize the urethra
• Urine can be evaluated for microorganisms,
tissue proteins, RBCs and WBCs


Specimens used in lab Dx
 Genital lesions

• Bacteria may be isolated directly from:
• Cutaneous lesions
• Fluid obtained from the lesions
• Pus
• Blood cultures may not be useful, although
serologic evaluations may be effective
• Identification of the agent that causes syphilis (T.
pallidum) cannot be performed using normal
bacterial culture methods
• Fluorescent darkfield examination
• Treponemal and non-treponemal serology tests


Specimens used in lab Dx
 Fecal specimens

• A large number of bacteria colonize the intestinal
• Many bacteria may cause gastroenteritis and can
be cultured
• Requires a stool sample
• Quantity of the sample needs to be adequate to culture
out the potential pathogen
• Culturing these bacteria may require different
culture methods or media
• Because culture may take > 2-3 days, treatment is
usually initiated empirically