Bacterial infections Flashcards
How are medically important bacteria classified
Conceptually, they are classified regarding their ability to retain crystal violet dye during Gram staining process (positive or negative) or method of energy production (aerobic cellular respiration or anaerobic glycolysis)
Cite the four steps of the Gram stain
- Step 1: crystal violet (primary stain), for 1 minute before rinsing
- Step 2: iodine (binds to form crystals) for 1 min before rinsing
- Step 3: acid/alcohol (decolorises Gram neg) until purple stain is just gone then rinse
- Step 4: safranin or basic fuschine (counnter stain) for 1-2 min then rinse
What characterizes Gram-positive bacteria
Gram-positive bacteria are characterized by a dinstinctive cell wall comprising a thick peptidoglycan layer (composed of sugars and amino acids)
This layer gives structural integrity to the bacteria and counteracts changes in osmotic pressure inside the cell
The peptidoglycan layer of Gram-positive bacteria is 20-80nm, compared to 7-8nm in Gram-negative bacteria
What characterizes Gram-negative bacteria
Gram-negative bacteria have a much more complex structure than Gram-positive bacteria
They have developed a second outer lipid bilayer membrane as well as a peptidoglycan layer
The outer layer contains a large amount of lipopolysaccharide which consists of a lipid portion within the membrane (lipid A) and a polysaccharide portion extending outwards from the surface of the bacteria (the O antigen)
The lipopolysaccharide portion of the outer Gram-negative bacterial membrane is otherwise known as endotoxin
- This is released when Gram-negative bacteria lyse
- It may induce a widespread inflammatory response (SIRS) and multiple organ dysfunction syndrome (MODS) if the endotoxin enters the systemic blood stream (endotoxemia)
- Fortunately, sepsis (characterized by bacteremia) and endotoxemia are quite uncommon in cats compared to dogs
Explain the difference between aerobic, facultative anaerobic and anaerobic bacteria
- Obligate anarobes cannot survive in the presence of oxygen
+ They cannot manufacture superoxide dismutase (and most cannot produce catalase) which are required to breakdown the toxic oxygen metabolites
+ Anaerobic bacteria are the most common species living on mucosal surfaces, therefore they make up a large proportion of the cat’s normal flora (e.g., Bacteroides, Fusobacterium)
+ They play an important role in regulating the numbers of aerobic bacteria (which tend to be more pathogenic) - Facultative anaerobes can survive, as they can switch between cellular respiration and anaerobic glycolysis
When do anaerobic bacteria become pathogenic and what changes do they induce into the tissues
- When anaerobic bacteria become pathogenic, it is generally because there has been a breach in the host’s normal defenses
- Generally, there is involvement of more than one species of anarobes in these infections
- Almost invariably, infections with anaerobic organisms induce a pyonecrotic change within affected tissues
What are the most common sites in which anaerobic infections are involved
- The pleural space (pyothorax)
- Bite wounds (cellulitis)
- Bone (osteomyelitis)
- The peritoneal cavity
- Peri-oral tissues
- Upper and lower respiratory tracts
What are the classical clinical signs characterizing infection with anaerobes
These infections are generally characterized by the presence of:
- pain
- fever
- swelling
- erythema
- foul smelling exudates
- +/- the formation of gas within the tissues
What is the classical therapy of infetion with anaerobes
Successful therapy often involves a combination of medical and surgical intervention (such as wound drainage)
Most of these infections are treated empirically with either metronidazole (10mg/kg, PO, q12h), amoxicillin-clavulanate (20mg/kg, PO, q8-12h) or clindamycin (5 mg/kg, PO, q12)
