Gram-Negative Cocci

Question 1

Describe the gram-staining characteristics of Neisseria spp.

Answer 1

Neisseria are gram-negative diplococci with adjacent sides flattened. They resemble coffee beans. In a direct smear, pathogenic Neisseria are intracellular.

Question 2

How do the growth requirements of pathogenic and nonpathogenic Neisseria differ?

Answer 2

The pathogens require an enriched medium (chocolate agar) and increased carbon dioxide. The nonpathogens grow on blood agar without increased carbon dioxide.

Question 3

Which specimens might yield pathogenic Neisseria?

Answer 3

CSF, blood, synovial fluid, and genital specimens. These specimens should not be refrigerated because pathogenic Neisseria are sensitive to low temperatures. When Neisseria is suspected, it is best to plate the specimens directly and to use a carbon dioxide generating transport device.

Question 4

Which species of Neisseria is always considered a pathogen?

Answer 4

N. gonorrhoeae. Some individuals carry N. meningitidis in the nasopharynx.

Question 5

Which Neisseria species are part of the normal flora?

Answer 5

N. lactamica, N. flavescens, N. sicca, N. subflava, and N. mucosa are normal in the upper respiratory tract. N. cinerea is normal flora of the oropharynx and genital tract.

Question 6

Name several gonococcal selective media.

Answer 6

Modified Thayer Martin (MTM), Martin Lewis, and New York City medium (NYC).

Question 7

Which specimens are plated on gonococcal selective media?

Answer 7

Specimens for GC screens from sites with normal flora such as genital specimens, throat swabs, and rectal swabs. Normal flora is inhibited.

Question 8

What organisms besides N. gonorrhoeae may grow on gonococcal selective media?

Answer 8

N. meningitidis, N. lactamica, N. cinerea, Moraxella (Branhamella) catarrhalis, and Kingella denitrificans.

Question 9

Which pathogenic Neisseria can grow on blood agar?

Answer 9

N. meningitidis. N. gonorrhoeae does not.

Question 10

Name several ways in which N. gonorrhoeae and N. meningitidis are alike.

Answer 10

Both are gram-negative diplococci, oxidase-positive, catalase-positive, and positive for glucose utilization.

Question 11

How can N. gonorrhoeae and N. meningitidis be differentiated?

Answer 11

By enzyme substrate tests, antigen detection tests, or tests of sugar utilization. N. meningitidis is positive for maltose and N. gonorrhoeae is negative.

Question 12

Which Neisseria causes salpingitis adn opthalmia of the newborn?

Answer 12

N. gonorrhoeae.

Question 13

What non-culture tests are available for detection of N. gonorrhoeae directly from clinical specimens?

Answer 13

Antigen detection tests, direct specimen nucleic acid probes, and nucleic acid amplification tests.

Question 14

What constitutes a definitive diagnosis of N. gonorrhoeae?

Answer 14

Colonies with typical colonial morphology
Oxidase-positive
Gram-negative diplococci
Confirmation by biochemical, enzymatic, serologic, or nucleic acid test

Question 15

Which gram-negative diplococci are normal in the upper respiratory tract and cause otitis media and sinusitis in children?

Answer 15

Moraxella (Branhamella) catarrhalis.

Question 16

Which rapid test can differentiate N. meningitidis from N. lactamica?

Answer 16

ONPG. N. lactamica is ONPG-positive; N. meningitidis is ONPG-negative.

Question 17

What are the CTA sugar reactions for N. gonorrhoeae, N. meningitidis, N. lactamica, and Moraxella catarrhalis?

Answer 17

N. gonorrhoeae: glucose (+), maltose (-), lactose (-)
N. meningitidis: glucose (+), maltose (+), lactose (-)
N. lactamica: glucose (+), maltose (+), lactose (+ slow)
M. catarrhalis: glucose (-), maltose (-), lactose (-)

Question 18

Which Neisseria produces rapid and inmediate bubbling with 30% hydrogen peroxide (superoxol test)>

Answer 18

N. gonorrhoeae. N. meningitidis adn N. lactamica produce weak delayed bubbling.

Question 19

What is PPNG?

Answer 19

Penicillinase-producing Neisseria gonorrhoeae. These penicillin-resistant organisms can be identified by a positive B-lactamase test.

Question 20

What organism might be mistaken for Neisseria on a Gram stain?

Answer 20

Moraxella catarrhalis, Kingella denitrificans, and Acinetobacter.

Question 21

Which of the following is oxidase-negative: Neisseria, Moraxella, Kingella, or Acinetobacter?

Answer 21

Acinetobacter.

Question 22

A gram-negative coccobacillus isolated from cerebrospinal fluid on chocolate agar resembles either Moraxella or Neisseria. What is the single best test to differentiate these two organisms?

Answer 22

Moraxella and Neisseria appear similar on a Gram stain, both are oxidase-positive, and both grow on gonococcal selective media. They can be differentiated by sugar utilization tests. Moraxella is negative for all the carbohydrates in the sugar utilization test. Neisseria is positive for at least one, depending on the species.