SOM 208 Medical Microbiology Syllabus
Bacteremia and Septic Shock.

…the micro-organisms that seem to have it in for us . . . turn out . . . to be rather more like
bystanders.

. . . It is our response to their presence that makes the disease.

Our arsenals for fighting

off bacteria are so powerful . . .

that we are more in danger from them than the invaders.

Lewis Thomas, The Lives of a Cell: Notes of a Biology Watcher, 1974

The normal response to bacterial infection is a complex inflammatory process that attempts to
localize the infection and repair the tissue. This response involves the activation of circulating and
fixed phagocytic cells and the generation of both pro-inflammatory and anti-inflammatory mediators.
A balance between these mediators helps to facilitate tissue healing. Sepsis results when the
balance is lost; the inflammatory response to an infection extends beyond the infected tissues, and
becomes generalized. The process to control the infection then becomes uncontrolled, unregulated
and self-sustaining. It is estimated that 750,000 cases of sepsis are diagnosed annually, and result
in more than 210,000 deaths despite antibiotics and intensive care.

Hey! The nomenclature for this section can get confusing. In 1992, the

American College of Chest

Physicians/Society of Critical Care Medicine Consensus Conference (Crit Care Med 1992
Jun;20(6):864-74)

agreed on the following definitions:

Systemic inflammatory response syndrome (SIRS) is a widespread inflammatory response to a variety
of severe clinical insults. This syndrome is clinically recognized by the presence of two or more of the
following

•

Temperature >38ºC or <36ºC

•

Heart rate >90 beats/min

•

Respiratory rate >20 breaths/min or PaCO2 <32 mmHg

•

WBC >12,000 cells/mm3, <4000 cells/mm3, or with >10 percent immature (band) forms.

Sepsis is the systemic response to infection. So if a patient has sepsis, they have the clinical signs of
SIRS with concrete evidence of infection.

Severe sepsis is associated with organ dysfunction, hypoperfusion, or hypotension. Clinical
manifestations of hypoperfusion may include lactic acidosis, oliguria, or an acute alteration in mental
status.

Septic shock is sepsis with hypotension despite adequate fluid resuscitation combined with perfusion
abnormalities that may include, but are not limited to, lactic acidosis, oliguria, or an acute alteration in
mental status. Patients who require inotropic or vasopressor support, despite adequate fluid resuscitation,
are in septic shock.

Hypotension is defined as a systolic BP of <90 mmHg or a reduction of 40 mmHg from baseline in the
absence of other causes for the fall in blood pressure.

Multiple organ failure is the presence of altered organ function in an acutely ill patient such that
homeostasis cannot be maintained without intervention.

Bacteremia is the presence of viable bacteria in the blood.

The Pathophysiology of Septic Shock

Epidemiology of Septic Shock

Septic shock is a clinical syndrome that is characterized by wide spread tissue injury and systemic inflammation. Clinical
signs and symptoms of septic shock include:

• A feeling of impending doom

• Hyperventilation

• Fever

• Hypotension due to low SVR

• Accumulation of lactic acid, although hyperventilation may mask the acidosis by lowering the pCO

2

• Organ damage- kidney, lung, liver, gut, heart This can be come profound and lead to organ failure.

• Disseminated intravascular coagulation

• Leukopenia --- Leukocytosis

SOM 208 Medical Microbiology Syllabus
Bacteremia and Septic Shock.

• Activation of the neuro-endocrine axis

• Cytokine induction – IL-1, Il-6, IL-10, Il-12, TNF, and others.

About 50% of patients with septic shock have bacteremia. In septic shock patients with a positive blood culture, the
following frequencies of organisms are seen:

• Gram-negative

rods

35%

• Gram-positive

organism 35%

• Mixed

Infection

19%

• Fungi 13%.

The source of the patient’s infection is usually endogenous. The most common sites of infections leading to septic shock
are the lungs, abdomen and urinary tract. The site of infection can be an important factor in predicting clinical outcome.
For example, severe sepsis is more likely to occur in nosocomial pneumonia than in the bacteremia acquired from an
indwelling urinary catheter.

Factors contributing to the increasing incidence of sepsis include aggressive oncological therapy, widespread use of
corticosteroid and immunosuppressive therapies, antimicrobial-resistant organisms, increased use of invasive devices,
and longer lives of patients predisposed to sepsis, such as the elderly, diabetics and cancer patients.

Clinical issues that relate to the diagnosis of sepsis and its management will be studied during the clinical years. This
discussion will be targeted at understanding the pathophysiology of septic shock.

Host Responses to Pathogens

A very complex sequence of events that is not completely understood leads to the development of septic shock in a
patient. Circulating proteins interact with microbial products initiating or accelerating a cascade of activation. Initially,
patients have an overwhelming pro-inflammatory response to an infection mediated by Tumor Necrosis Factor (TNF),
interferon gamma, IL-1, IL-6, and IL-12. In addition,

complement

is activated.

Coagulation proteins

are activated by

tissue factor expression on macrophages and endothelial cells.

Fibrinolysis is inactivated

by the loss of activated protein

C and thrombomodulin on endothelial cells

. iNOS is transcriptionally activated

in endothelial cells & elsewhere

leading to

relaxation of tone of arteriolar smooth muscle

. The body then attempts to mitigate this response by producing anti-

inflammatory cytokines, such as IL-10, and soluble inhibitors such as soluble TNF receptors, IL-1 receptor type II, and IL-1
RA (receptor antagonist) (an inactive form of IL-1). In sepsis, the pro-inflammatory response is dominant, especially
early, and coagulation is dominant over fibrinolysis. This results in disseminated intravascular coagulation, ischemia,
hypoperfusion and tissue injury. As time goes on, the anti-inflammatory responses become dominant and apoptosis of
immune cells makes the patient more susceptible to opportunistic infections.

The complications of this uncontrolled cascade include:

• Multi-organ

failure

o

Lungs, which can manifest as Acute Respiratory Distress Syndrome (ARDS)

o

Kidneys, which can manifest as acute tubular necrosis (ATN)

o

Liver, leading to hepatitis

o

Heart, leading to decreased contractility

o

Brain, leading to confusion

•

Disseminated intravascular coagulation

• Death

o

There is a 30-50% mortality from septic shock.

Pathogen Associated Molecular Patterns (PAMPs) and PAMP Receptors

Many bacterial products are known to set off the systemic cascade that can result in septic shock. These include:

•

Lipopolysaccharide

(LPS) or endotoxin

o

Endotoxin is the most common and potent of the products that can potentiate sepsis.

o

Remember from the Bacterial Structure notes that LPS serves as a lipid barrier for Gram negative
organisms that lack a thick peptidoglycan layer.

o

Lipopolysaccharide consists of lipid A, the hydrophobic moiety, a highly conserved inner core, and a
highly variable “O” side chain composed of 100-1000 sugars.

o

The lipid A region, which is highly conserved, is the toxic portion of this PAMP.

• Peptidoglycan

• Bacterial

lipoproteins

SOM 208 Medical Microbiology Syllabus
Bacteremia and Septic Shock.

•

Mycobacterium tuberculosis lipoproteins and lipoarabinomannan

• Flagellin

•

Heat shock proteins

LPS has a complex set of receptors:

•

LPS binding protein is a 60kD acute phase

plasma

protein that tightly binds LPS, and enhances the cell

response to LPS. It is usually present in approximately 5

µg/ml in normal individuals. In sepsis, it level may be

elevated to > 100

µg/ml. It catalyzes the binding of LPS to CD14.

•

CD14 is a 55 kD glycoprotein that is present in both the cell surface and plasma (sCD14). It has a wide
spectrum of binding activity, and will bind among other molecules LPS, peptidoglycan, and heat shock proteins.
It is considered a pattern recognition receptor that evolved because of the need for an immediate response to
invasion. CD14 is a

glycophosphatidylinositol (GPI)-

anchored protein expressed on the surface of macrophages

and neutrophils. It has no transmembrane domain, and doesn’t transmit an intracellular signal.

The signal transducing receptor for LPS is toll-like receptor 4 (TLR-4).

•

It forms complexes with CD14 and a TLR-4 associated extracellular molecule known as MD-2. Toll-Like
receptors (TLRs) play a critical role in early innate immunity to invading pathogens by sensing
microorganisms. (The Toll gene is found in Drosophila melanogaster, the fruit fly. Genes that encode Toll-
like receptors are highly conserved between species.)

•

Binding of TLR-4 to the LPS causes phophorylation of specific transmembrane tyrosines, which activate
intracellular signaling cascades leading to transcriptional activation of many genes.

There is a family of TLRs, TLR 1-9:

•

TLR-2 interacts with peptidoglycan, bacterial lipoproteins, lipoteichoic acid, fungal glucan.

•

TLR-4 interacts with LPS, heat shock proteins.

•

TLR-5 interacts with flagellin.

Many cells respond in sepsis, including:

• Macrophages

o

Once macrophages recognize microbial products, they activate NF-

κB, which transcriptionally regulates

gene expression of CD14, TLRs and many cytokines.

o

Over 100 genes are transcriptionally activated by microbial products.

o

Macrophages then secrete many mediators including IL-1, IL-6, IL-8, IL-10, IL-12, GMCSF, TNF,
reactive O

2

intermediates, platelet activating factor and leukotrienes.

o

There is an upregulation of MHC, ICAM, VCAM and tissue factor.

o

There is upregulation of the respiratory burst through the induction of inducible nitric oxide synthetase
(iNOS).

• PMNs

• Endothelial

cells

o

Soluble CD14 appears to be required for an endothelial cell response.

o

Inflammation and cytokines like IL-1 and TNF causes endothelial cells to produce intergrins such as
ICAM, VCAM and ELAM.

o

Endothelial cells also produce cytokines like IL-1, IL-6, TNF, and IL-8.

o

Upregulation of iNOS leads to the production of NO which diffuses to arteriolar smooth muscle causing
relaxation of tone and low SVR.

o

Other mediators are produced such as endothelin, and prostaglandins.

o

A loosening of the tight junctions and apoptosis can also be seen. The apoptosis of endothelial cells
can lead to irreversible organ damage.

• Epithelial

cells

o

There is an upregulation of VCAM, ICAM etc.

o

Production of IL-1 and TNF activates a positive feedback loop.

o

Production of IL-6 is seen.

o

Bladder and respiratory epithelial cells can all respond to LPS.

Treatment

It is difficult to design a therapy for septic shock. Some factors which confound its study include:

•

It is difficult to design a good sepsis trial. Patients are a heterogeneous group, and there are multiple triggers.

•

Therapy for one microbial trigger may not work for others.

•

Multiple cytokines are involved, so blocking one has little effect.

SOM 208 Medical Microbiology Syllabus
Bacteremia and Septic Shock.

Over the past 10 years numerous therapies have been tried, however, they failed to improve clinical outcome, and
sometimes increased mortality. Hence, septic shock remains a major source of morbidity and mortality. These therapies
included:

• Anti-inflammatory

medications

•

Antiendotoxin therapy (Monoclonal antibodies to the lipid A region of LPS)

•

Anticytokine therapy (Inhibitors of IL-1, and TNF)

• iNOS

inhibitors.

Medical Progress: The Pathophysiology and Treatment of Sepsis
Hotchkiss R. S., Karl I. E.

N Engl J Med 2003; 348:138-150, Jan 9, 2003.