RSV

This virus is extremely common with most people being infected at least once before they reach two years old.

While most RSV infections are mild, it is estimated that worldwide more than 3.2 million people per year are hospitalized due to RSV infection.

2023 saw two exciting developments in managing this infection: a vaccine (Arexvy) for older adults and a monoclonal antibody (nirsevimab or Beyfortus) for young children.

MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) was first detected in 1961, only one year after methicillin started being used.

Sulfamethoxazole/trimethoprim (Bactrim or Septra) and doxycycline are reliable oral antibiotics for MRSA infections.

Clindamycin does have activity against MRSA but because it must be taken three or four times a day, is one of the most common causes of Clostridioides difficle diarrhea, and about 25% of MRSA in the area is resistant to it, clindamycin should be reserved for patients who cannot tolerate other options.

CRE

Thankfully, carbapenem-resistant Enterobacterales (CRE) are not very common in the United States (less than 15,000 infections annually - compare this to more than 550,000 catheter-associated UTIs every year).

Severe infections caused by CRE usually have very limited antibiotic options, which may include cefiderocol (Fetroja) or ceftazidime/avibactam (Avycaz) + aztreonam.

In the U.S., most CRE is associated either with extensive exposure to antibiotics or recent travel to areas of the world where CRE is common.

Rickettsia rickettsii

Rickettsia rickettsii is an intracellular Gram negative coccobacillus (shaped somewhere between a sphere and a rod) spread via tick bite that was first discovered in 1902 by Howard Ricketts at the University of Chicago.

Prior to the advent of antibiotics, Rocky Mountain Spotted Fever (RMSF) was deadly - almost 70% of those with RMSF died from the infection.

While today RMSF is more common on the U.S.'s East coast, it was originally seen in Idaho and called the Black Measles because of how the skin looked late in the disease.

Pseudomonas aeruginosa

This Gram negative, non-lactose-fermenting rod is a common cause of hospital-onset infections.

The only oral treatment options are fluoroquinolones, though moxifloxacin should not be used without proven susceptibility testing.

Pseudomonas aeruginosa can have a green sheen when grown on agar, looking like an oil slick.

This organism is not part of the human normal flora, which is why risk factors are useful when determining which patients should receive empiric coverage for Pseudomonas infections.

Pseudomonas is a water-loving organism that can find a home in shoes, showers, and live plants.

VRE

Vancomycin-resistant Enterococci are very difficult to treat organisms, frequently only susceptible to linezolid or daptomycin.

Thankfully, VRE in the urine is frequently still susceptible to nitrofurantoin, making this antibiotic a good option for patients with UTIs.

These Gram positive cocci usually appear when a person has extensive antibiotic exposure, which kills off all the normal flora until VRE is one of the only organisms left. Enterococcus species usually reside in the human GI tract and are inherently resistant to cephalosporins.

MSSA

Methicillin-susceptible Staphylococcus aureus (MSSA) can cause the same kinds of severe infections that are caused by MRSA. This is why Infectious Disease physicians should be involved in the care of patients with Staphylococcus aureus in their blood, regardless of if it is MRSA or MSSA.

MSSA can be treated with many more kinds of antibiotics than MRSA. Beta-lactam antibiotics like anti-Staphylococcal penicillins (oxacillin, dicloxacillin, nafcillin) and first generation cephalosporins (cefazolin, cephalexin, cefadroxil) are first-line treatments for MSSA that are inactive against MRSA.

Klebsiella aerogenes

Enterobacter species are known for rapidly developing resistance to penicillins and early generation cephalosporins whereas Klebsiella species are not.

Enterobacter aerogenes was recently renamed Klebsiella aerogenes based on genetic findings from whole-genome sequencing; however, despite its new name, this organism continues to have the same issues with resistance development and must be considered differently than other Klebsiella species.

For example, a severe infection with Klebsiella pneumoniae may be treated with ceftriaxone but a severe infection with Klebsiella aerogenes may need cefepime instead of ceftriaxone to optimize therapy.

ESBL E. coli

Extended-spectrum beta-lactamase (ESBL) is a kind of enzyme produced by bacteria to break down beta-lactam antibiotics like penicillins, cephalosporins, and aztreonam.

Severe infections caused by bacteria with ESBLs are usually treated with carbapenems like meropenem or ertapenem, but drugs from non-beta-lactam classes like fluoroquinolones (levofloxacin, ciprofloxacin), aminoglycosides (gentamicin, tobramycin), sulfamethoxazole/trimethoprim (Bactrim, Septra), and nitrofurantoin may still be helpful if susceptible.

ESBLs are common in people who have been extensively exposed to antibiotics, particularly with long courses of beta-lactam antibiotics like ceftriaxone.

Candida albicans

This yeast is the most common species of Candida to cause infections in humans.

It is (almost) always susceptible to fluconazole.

C. albicans is part of the human normal flora. Since antibiotics are not active against this yeast, killing off a human's bacteria can lead to yeast overgrowth and infection (i.e., thrush, vaginal yeast infections, fungemia).

It is vanishingly rare to have a Candida pneumonia. If you see a yeast growing in a respiratory culture, it is very likely a contaminant from the patient's mouth or esophagus that occurred during specimen collection.

Candida species like the eyes. It was previously recommended that any patient with candidemia undergo an eye exam to rule out endophthalmitis, but recommendations are slowly changing to include only patients who report symptoms or are unable to report symptoms.

Clostridioides difficile

This Gram positive anaerobe also known as C. diff has very few treatment options and can be devasting to a patient's quality of life.

The most common cause of C. difficile infections is recent use of antibiotics, but other risk factors contribute: age 65+ years old, recent hospitalization or nursing home stay, weakened immune system, or history of C. diff.

Previously "Clostridium" difficile, this organism produces two toxins (A and B) that cause havoc in the gut. Other Clostridium species are also known for their toxins and cause diseases such as gangrene, botulism, and tetanus.

Testing has changed over time. PCR-based testing looks for the gene that can produce the toxin, not the toxin itself, so PCR-based testing can over-diagnose the infection. This is why many institutions utilize a two-step process, first looking for toxin, then looking for the gene.

C. diff infection almost always affects the colon. Small bowel involvement is extremely rare. If a patient does not have a colon, they are unlikely to get C. diff.

HIV

HIV is a single-stranded RNA virus that uses the human body to copy its RNA into DNA using the enzyme reverse transcriptase in CD4 lymphocytes, slowly destroying these cells. HIV is generally not curable (though several people have cleared infection after stem cell transplants). This is mainly due to the latent HIV reservoir - antiretrovirals cannot clear the reservoir, so this is a chronic, though manageable, infection.

Always double check drug-drug interactions when patients are prescribed antiretrovirals. Some great references are the Liverpool HIV Drug Interactions Checker ( hiv-druginteractions.org ) and the Drug-Drug Interactions section of the DHHS guidelines ( clinicalinfo.hiv.gov ).

The first cases in the U.S. were reported in the 1980s, and zidovudine was the first antiretroviral therapy available in 1987. Using multiple agents did not become the standard of care until the mid-1990s. In 2006, Atripla became the first one-pill once daily regimen.

The virus itself does not kill patients; rather, opportunistic infections occurring after the virus depletes a patient's immune system are what prove fatal.

Treating HIV and getting viral loads to undetectable levels is very effective in preventing transmission. This is where the treatment as prevention, or undetectable = untransmittable (U=U), initiative came from.

Syphilis

Treponema pallidum is the spirochete bacterium that causes syphilis. T. pallidum has been infecting humans for thousands of years. Infections have increased rapidly in the U.S. in recent years, including a doubling of mother-to-baby transmission between 2017 and 2021.

This organism is smaller than the average bacterium (only 20-25% the size of E. coli) and tends to evade the immune system, leading to one nickname “the stealth pathogen.” It can present in many different forms, leading to other nicknames such as “the great pretender” and “the great imitator.”

Penicillin is first-line treatment, though ceftriaxone and doxycycline have been used for patients with true penicillin allergies.

Streptococcus pneumoniae

This Gram positive cocci is famous for being in lancet-shaped pairs. This is why one of its nicknames is "the Diplococcus."

Not only is this an extremely common cause of community-acquired pneumonia (hence "pneumoniae" as the species name), but it is also an extremely common cause of community-acquired meningitis.

S. pneumoniae is extremely susceptible to penicillin, even if susceptibility testing indicates intermediate susceptibility.

Another S. pneumoniae nickname is "The captain of the men of death."

Streptococcus pneumoniae is an alpha-hemolytic strep species, unlike the beta (groups A, B, C, etc) or gamma streps (Enterococcus species, S. gallolyticus).

Streptococcus pyogenes

This Gram positive cocci can be found in chains and is also known as Group A Strep (often abbreviated GAS).

It is a common cause of Strep throat and toxic shock syndrome.

S. pneumonmiae is normal flora of the upper respiratory tract, genital mucosa, and skin, with 2-3% of adults colonized with it.

ALL GAS is susceptible to penicillin; therefore, it is first-line therapy and does not require susceptibility testing.

Traditionally, clindamycin has been added to other antibiotic therapy for severe GAS infections (i.e., toxic shock) to suppress toxin and cytokine production. However, recently, linezolid is frequently being used instead.

Staphylococcus lugdunensis

This Gram positive cocci exists in clusters and is a species of coagulase negative Staphylococcus (CONS).

This organism is known to be as destructive as Staphylococcus aureus so should be taken as seriously.

S. lugdunensis is the second most common cause of CONS endocarditis after Staphylococcus epidermidis and has a predilection for native valves over prosthetic valves.

Unlike S. aureus, it is most commonly methicillin susceptible. mecA gene presence indicates methicillin resistance, same as with S. aureus.

The frequency of S. lugdunensis infections is likely under-appreciated since not all laboratories differentiate it from other CONS species.