Mycoplasma genitalium (Mg) was first isolated from 2 of 13 men with nongonococcal urethritis (NGU) in 1982. Though subsequent attempts to culture the organism from men with NGU in the 1980's failed because of its fastidious growth requirements. Now in recent studies it make it presense clearly even stronger than gonorrhea.

Study: Relatively new STD eclipses gonorrhea                                              Infection prevalence was 11 times higher in cohabiting couples, study finds http://www.msnbc.msn.com/id/19101160/

My test is negative for chlamydia and gonorrhea, could I have NGU ?

Yes, you could. NGU causes other than chlamydia is getting more common. Just in our clinical practice in Southern California it is the most comon type of urethritis (other than Chlamydia).  

Symptoms of non-gonococcal urethritis

Up to 1/3 of men presenting at an STD clinic with laboratory confirmed NGU have no signs (discharge) or symptoms (dysuria/itching).

• A white discharge  from the end of the penis, but does not occur in every case.
• Pain or burning when you pass urine. This may be confused with a urine infection.
• Irritation
• urinary frequency
• tingling 

Complication If Untreated:  NGU may facilitate HIV transmission. Patients who have NGU and also are infected with HIV should receive the same treatment regimen as those who are HIV-negative.

Untreated NGU may lead to

Specifics

1. Chlamydia- in some areas of the US the proportion of NGU due to this organism is declining perhaps as a result of increased screening efforts.
2. Ureaplasma- urethral colonization in men without urethritis is common but has been associated with clinical urethritis in men without any other discernible cause. However, the role of this organism in clinical disease remains unclear.
3. Mycoplasma genitalium- isolated more frequently in men with NGU than from those without but the causal role is still not proven.
4. Trichomonas- implicated in higher proportion on NGU cases in eastern Europe, Africa, and India compared with the US. Recent data suggests, however, that trichomoniasis is increasing as a cause of NGU (about 10%). Incidence of trichomonal NGU may be higher in men more than 30 years of age.
5. HSV- Urethritis in 15-30% of men with primary HSV. Less frequent with recurrent HSV. Most, but not all patients with HSV urethritis have visible penile lesions.
6. Yeast- rarely detected in urethritis study but some men with balanitis have symptoms of urethritis.
7. Urethral gram stain may demonstrate WBC and gram negative rods. Maybe associated with cystitis and occasionally with insertive anal sex. Treatment should be directed towards gram negative rods.
8. Noninfectious causes of urethritis- trauma, chemical irritation, Steven-Johnson syndrome."  Boston University education