Treatment for Shigella Infection
Anyone with a suspected Shigella infection should be tested for the presence of Shigella bacteria. Antibiotics can shorten the length of illness.
Although shigellosis is usually a self-limited illness, antibiotics can shorten the course, and in the most serious cases, might be life-saving. [1, 16, 22] Historically, the antibiotics commonly used for treatment of bacterial infections, like those caused by Shigella, are ampicillin, trimethoprim/sulfamethoxazole (TMP-SMZ, also known as Bactrim or Septra), or ceftriaxone (Rocephin). [1, 11, 26] Ciprofloxacin is also used commonly to treat adults who are infected. [11, 26, 30].
Unfortunately, Shigella bacteria have become resistant to one or more of these antibiotics. [16, 30] This means some antibiotics might not be effective for treatment, and that using (or overusing) antibiotics to treat shigellosis can sometimes make the bacteria more resistant.  As noted in one recent study:
Of 369 isolates tested, 59% were resistant to TMP-SMZ, 63% were resistant to ampicillin, 1% were resistant to cefixime, and 0.3% were resistant to nalidixic acid; none of the isolates were resistant to ciprofloxacin. Thirteen percent of the isolates had multidrug resistance to ampicillin, chloramphenicol, streptomycin, sulfisoxazole, and tetracycline. Infections due to multidrug-resistant shigellae are endemic in Oregon. 
This study therefore suggests that “[n]either ampicillin nor TMP-SMZ should be considered appropriate empirical therapy for shigellosis any longer; when antibiotics are indicated, a quinolone or cefixime should be used.”