Biofilms and Implant-Related Infections — MCQs
Oil paint appearance on nutrient agar is seen in -
What is the best management for a human bite?
A study of nosocomial infections involving urinary catheters is performed. The study shows that the longer an indwelling urinary catheter remains, the higher the rate of symptomatic urinary tract infections (UTIs). Most of these infections are bacterial. Which of the following properties of these bacteria increase the risk for nosocomial UTIs?
Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?
The most common mechanism of resistance to drugs in Staphylococcus is
Among the following organisms, which is the most common cause of postoperative endophthalmitis following corneal transplantation?
Immediate treatment of compound fracture of tibia includes:
Which of the following statements about Staphylococcus epidermidis is true?
Which is a minor criterion for diagnosis of RF according to modified Jones criteria?
A patient presented with 70% burns, and a sample was collected from the burn site. The image shows Gram-negative rods, and the suspected organism is an obligate aerobe. What is the most likely causative microbe?
Biofilms and Implant-Related Infections Indian Medical PG Practice Questions and MCQs
Question 1: Oil paint appearance on nutrient agar is seen in -
- A. Staphylococcus aureus (Correct Answer)
- B. Streptococcus pyogenes
- C. Bordetella pertussis
- D. H. influenzae
Explanation: ***Staphylococcus aureus*** - *Staphylococcus aureus* forms characteristic **golden-yellow, smooth, opaque colonies** on nutrient agar with a **buttery or creamy consistency** - Some texts describe this appearance as **"oil paint-like"** due to the pigmented, smooth, and glistening surface that can resemble brushed paint - Colonies are typically **2-4 mm in diameter**, round, and show **golden pigmentation** (due to carotenoid pigments) - On **blood agar**, *S. aureus* shows **beta-hemolysis** with golden colonies *Streptococcus pyogenes* - *Streptococcus pyogenes* grows poorly on plain nutrient agar and requires **enriched media** like blood agar - On blood agar, it forms **small, translucent, grey-white colonies** surrounded by a wide zone of **beta-hemolysis** - Colonies are typically **pinpoint** in size and do not show pigmentation *Bordetella pertussis* - *Bordetella pertussis* is a **fastidious organism** that does **not grow on plain nutrient agar** - Requires specialized enriched media like **Bordet-Gengou agar** (with potato-glycerol-blood) or **Regan-Lowe agar** - On Bordet-Gengou agar, colonies appear as **small, smooth, pearl-like** or **"mercury droplet"** colonies after 3-7 days *H. influenzae* - *Haemophilus influenzae* is also fastidious and requires **X factor (hemin)** and **V factor (NAD)** for growth - Does **not grow on plain nutrient agar** - On **chocolate agar**, forms **small, smooth, translucent, greyish colonies** with a characteristic musty odor - Colonies are typically **1-2 mm** in diameter
Question 2: What is the best management for a human bite?
- A. Ampicillin plus sulbactam (Correct Answer)
- B. Clindamycin plus TMP-SMX
- C. Fluoroquinolone
- D. Doxycycline
Explanation: ***Ampicillin plus sulbactam*** - This combination is effective against the common **aerobic and anaerobic bacteria** found in human bite wounds, including **Eikenella corrodens** and oral streptococci. - The sulbactam component provides **beta-lactamase inhibition**, which is crucial as many oral bacteria produce these enzymes, rendering ampicillin alone ineffective. *Clindamycin plus TMP-SMX* - While clindamycin covers many anaerobes, it has **poor activity against Eikenella corrodens**, a key pathogen in human bites. - **TMP-SMX (trimethoprim-sulfamethoxazole)** also lacks reliable coverage against many oral anaerobes and Eikenella. *Fluoroquinolone* - **Fluoroquinolones** generally have good Gram-negative coverage but often possess **limited activity against oral anaerobes and streptococci** relevant to human bites. - There is a **growing concern for resistance** with fluoroquinolone monotherapy in these types of infections. *Doxycycline* - Doxycycline has a broad spectrum but is **not the first-line choice for human bites** due to inconsistent activity against common oral anaerobes and Eikenella corrodens. - It may be considered in specific cases, but **empiric coverage needs to be broader** for initial management of these **polymicrobial infections**.
Question 3: A study of nosocomial infections involving urinary catheters is performed. The study shows that the longer an indwelling urinary catheter remains, the higher the rate of symptomatic urinary tract infections (UTIs). Most of these infections are bacterial. Which of the following properties of these bacteria increase the risk for nosocomial UTIs?
- A. Enzyme elaboration
- B. Biofilm formation (Correct Answer)
- C. Quorum sensing
- D. Exotoxin release
Explanation: ***Biofilm formation*** - **Biofilms** are communities of bacteria encased in a self-produced extracellular polymeric substance, adhering to surfaces like indwelling catheters. - The formation of a biofilm protects bacteria from antibiotics and host immune responses, allowing them to persist and proliferate, significantly increasing the risk of **catheter-associated urinary tract infections (CAUTIs)** over time. *Enzyme elaboration* - While some bacterial enzymes (e.g., urease) can contribute to UTI pathogenesis by increasing urine pH and promoting stone formation, it is not the primary property increasing the *risk* of nosocomial UTIs related to catheter duration. - The elaboration of various enzymes is a general virulence factor but doesn't specifically explain the increased risk due to the *presence* of a foreign body like a catheter. *Quorum sensing* - **Quorum sensing** is a system of stimuli and response correlated to population density, allowing bacteria to coordinate gene expression in response to their population density. - While quorum sensing plays a role in regulating virulence factors and biofilm maturation, it is a mechanism *within* a biofilm or bacterial population rather than the direct property of bacteria that increases the basal risk of infection on a catheter. *Exotoxin release* - **Exotoxins** are proteins secreted by bacteria that can cause damage to host cells and tissues, leading to specific disease symptoms (e.g., tetanus toxin, botulinum toxin). - While some exotoxins can contribute to the severity of infections, they are not the primary reason for the increased incidence of UTIs specifically due to the presence of an indwelling catheter; the physical presence of the catheter primarily promotes bacterial adhesion and persistence via means such as biofilm formation.
Question 4: Which of the following disorders would be more likely associated with Staphylococcus saprophyticus rather than Staphylococcus aureus?
- A. Burns
- B. Tension pneumothorax
- C. Osteomyelitis
- D. Acute cystitis (Correct Answer)
Explanation: ***Acute cystitis*** - **Staphylococcus saprophyticus** is a common cause of **urinary tract infections (UTIs)**, particularly acute cystitis, in young sexually active women. - This bacterium has a high affinity for **uroepithelial cells**, facilitating its colonization and subsequent infection of the bladder. *Tension pneumothorax* - A **tension pneumothorax** is a medical emergency characterized by air accumulation in the pleural space, leading to lung collapse and mediastinal shift. - It is typically caused by trauma or iatrogenic factors, not directly by bacterial infection from either *Staphylococcus saprophyticus* or *Staphylococcus aureus*. *Burns* - Burn wounds are highly susceptible to bacterial colonization and infection, with **Staphylococcus aureus** being a primary pathogen in this context. - *Staphylococcus saprophyticus* is rarely associated with burn wound infections. *Osteomyelitis* - **Osteomyelitis**, an infection of the bone, is most frequently caused by **Staphylococcus aureus** via hematogenous spread or direct inoculation. - *Staphylococcus saprophyticus* is not a common pathogen in osteomyelitis.
Question 5: The most common mechanism of resistance to drugs in Staphylococcus is
- A. Transformation
- B. Transduction (Correct Answer)
- C. Episomes
- D. Conjugation
Explanation: ***Correct Option: Transduction*** - **Transduction** is the transfer of genetic material via **bacteriophages** and is the **most common mechanism** of horizontal gene transfer in *Staphylococcus aureus*. - Bacteriophages play a crucial role in disseminating **antibiotic resistance genes** in staphylococci, including genes for **methicillin resistance (mecA)**, **toxins**, and **beta-lactamase**. - Phage-mediated transfer is responsible for spreading many **virulence factors** and **resistance determinants** among staphylococcal populations. *Incorrect Option: Episomes* - **Episomes** are plasmids capable of integrating into the bacterial chromosome or existing autonomously. - While episomes can **carry resistance genes**, they are a **genetic element**, not a **mechanism of transfer**. - The question asks about the mechanism, not the vehicle carrying resistance genes. *Incorrect Option: Transformation* - **Transformation** involves uptake of **naked DNA** from the environment. - *Staphylococcus* species are **not naturally competent** for transformation under normal conditions. - This is not a significant mechanism of resistance acquisition in staphylococci. *Incorrect Option: Conjugation* - **Conjugation** requires direct cell-to-cell contact through a **conjugative pilus**. - While possible in *Staphylococcus*, it is **less common** compared to transduction. - Conjugation is more characteristic of **Gram-negative bacteria** and enterococci among Gram-positives.
Question 6: Among the following organisms, which is the most common cause of postoperative endophthalmitis following corneal transplantation?
- A. Streptococcus
- B. Pseudomonas
- C. Propionibacterium acnes
- D. Staph epidermidis (Correct Answer)
Explanation: ***Staph epidermidis*** - **Coagulase-negative Staphylococcus** (CoNS), including *S. epidermidis*, is the most frequent cause of **postoperative endophthalmitis** after both cataract surgery and corneal transplantation due to its presence on the normal ocular flora. - These organisms can form **biofilms on intraocular lenses** or transplanted corneal tissue, making eradication difficult. *Streptococcus* - While *Streptococcus* species can cause endophthalmitis, they are associated with a **more virulent and rapid onset** of severe inflammation and are not the most common causative agents of postoperative endophthalmitis compared to *S. epidermidis*. - They tend to cause more aggressive infections with often **poorer visual outcomes**. *Propionibacterium acnes* - *Propionibacterium acnes* can cause a **late-onset, indolent form of endophthalmitis**, typically months or even years after surgery. - While it is a recognized cause, it is far **less common** than *Staphylococcus epidermidis* in immediate or early postoperative cases. *Pseudomonas* - **Pseudomonas aeruginosa** is an aggressive and rapid-onset pathogen often associated with **severe keratitis** or **post-traumatic endophthalmitis**. - Although it can cause postoperative endophthalmitis, it is **much less common** than coagulase-negative staphylococci due to its infrequency on normal conjunctival flora.
Question 7: Immediate treatment of compound fracture of tibia includes:
- A. Antibiotics and debridement only
- B. Debridement and splinting only
- C. Antibiotics, debridement, and splinting (Correct Answer)
- D. Debridement, splinting, and blood transfusion
Explanation: ***Antibiotics, debridement, and splinting*** - **Antibiotics** are crucial to prevent infection in **compound (open) fractures** due to communication with the external environment. - **Debridement** removes contaminated and devitalized tissue, while **splinting** stabilizes the fracture and minimizes further soft tissue damage. *Antibiotics and debridement only* - While antibiotics and debridement are essential, **splinting** is also critical for immobilizing the fracture and preventing further injury. - Without stabilization, the fracture site can move, causing additional soft tissue damage and increasing the risk of infection and delayed healing. *Debridement and splinting only* - This option overlooks the critical need for **antibiotics** in compound fractures, which are at high risk of infection due to the exposure of bone and tissue to bacteria. - Infection can lead to serious complications such as **osteomyelitis**, significantly impacting recovery and patient outcomes. *Debridement, splinting, and blood transfusion* - While debridement and splinting are correct, a **blood transfusion** is generally not an immediate routine treatment for all compound tibial fractures unless there is significant hemorrhage leading to hypovolemic shock. - The primary immediate concerns are infection prevention and stabilization, not typically massive blood loss requiring transfusion in every case.
Question 8: Which of the following statements about Staphylococcus epidermidis is true?
- A. The only coagulase negative staphylococcus of clinical significance
- B. Produces exotoxins
- C. Sensitive to methicillin
- D. Forms biofilms on medical devices (Correct Answer)
Explanation: ***Forms biofilms on medical devices*** - *Staphylococcus epidermidis* is a common cause of **nosocomial infections** associated with medical devices such as catheters, prosthetic joints, and heart valves. - Its ability to form **biofilms** allows it to adhere to these surfaces, evade host defenses, and resist antibiotic treatment. *Sensitive to methicillin* - While some strains of *S. epidermidis* may be sensitive, many strains are **methicillin-resistant** (MRSE), which is a significant clinical concern. - MRSE owes its resistance to the **_mecA_ gene**, which encodes for an altered penicillin-binding protein (PBP2a). *The only coagulase negative staphylococcus of clinical significance* - Although *S. epidermidis* is the most common and clinically significant **coagulase-negative staphylococcus (CoNS)**, other CoNS species, such as *Staphylococcus saprophyticus* (a cause of UTIs) and *Staphylococcus lugdunensis* (can cause endocarditis), are also clinically significant. - The classification "coagulase-negative" simply distinguishes them from *Staphylococcus aureus*, which produces coagulase. *Produces exotoxins* - While *S. aureus* is known for producing a wide array of potent **exotoxins** that contribute to its pathogenicity (e.g., toxic shock syndrome toxin, exfoliatin), *S. epidermidis* generally does not produce significant exotoxins. - Its pathogenicity primarily stems from its ability to form **biofilms** and its resistance to antibiotics.
Question 9: Which is a minor criterion for diagnosis of RF according to modified Jones criteria?
- A. Past History of Rheumatic Fever
- B. Fever (Correct Answer)
- C. Subcutaneous nodules
- D. ASO titre
Explanation: ***
Question 10: A patient presented with 70% burns, and a sample was collected from the burn site. The image shows Gram-negative rods, and the suspected organism is an obligate aerobe. What is the most likely causative microbe?
- A. Neisseria meningitidis (Meningococcus)
- B. Streptococcus pneumoniae (Pneumococcus)
- C. Pseudomonas aeruginosa (Correct Answer)
- D. Streptococcus pyogenes
Explanation: ***Pseudomonas aeruginosa*** - The image shows **Gram-negative rods**, and the patient has extensive **burns**, making *Pseudomonas aeruginosa* a highly likely causative agent due to its common association with burn wound infections. - *Pseudomonas aeruginosa* is an **obligate aerobe** and thrives in moist environments, making it a frequent colonizer of burn wounds, which are large, often moist surfaces. *Neisseria meningitidis (Meningococcus)* - *Neisseria meningitidis* is a **Gram-negative coccus**, typically appearing as diplococci, not rods, on Gram stain. - While it can cause severe infections, it is primarily associated with **meningitis** and **sepsis**, not typically burn wound infections. *Streptococcus pneumoniae (Pneumococcus)* - *Streptococcus pneumoniae* is a **Gram-positive coccus**, appearing as lancet-shaped diplococci or short chains, which contradicts the Gram-negative rod morphology seen in the image. - It is a common cause of **pneumonia** and **otitis media**, not primarily associated with burn wound infections. *Streptococcus pyogenes* - *Streptococcus pyogenes* is a **Gram-positive coccus** that grows in chains, which is inconsistent with the Gram-negative rod morphology. - Although it can cause skin infections like cellulitis and impetigo, it is not a typical cause of **burn wound infections** in the way *Pseudomonas aeruginosa* is.
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