Bacterial infections are the commonest cause of mastitis and are most frequent during lactation and pregnancy. Staphylococcus aureus is the usual cause of bacterial abscesses, including those that occur during lactation (Fig. 3.1). A minor (approximately 10%) proportion of S. aureus that cause mammary abscess are due to the methicillin-resistant variety (1), and antibiotic use ought to be guided by results of bacterial cultures.

Rare instances of mammary abscesses due to Nocardia (2), Salmonella (3,4,5), Pseudomonas (6), and brucellosis (7) have been reported. Mammary lesions owing to cat-scratch disease caused by Bartonella present as a mass with inflammatory signs, often accompanied by axillary nodal enlargement that may mimic inflammatory carcinoma (8). The mammary lesion of bartonellosis is in an intramammary lymph node rather than in mammary glandular or stromal parenchyma.

The existence of an immunodeficiency or immunocompromised state (including HIV/AIDS) can predispose to infections including those caused by Salmonella and Pseudomonas aeruginosa (9). Mammary gangrene has been reported in HIV-positive patients in the absence of prior trauma or other injury to the breast (10). Among other predisposing factors for mammary bacterial infections are the performance of needle core biopsy (11) and insertion of nipple rings purportedly for adornment purposes (12).

Actinomycotic infection of the breast typically presents as an abscess near the nipple and areola. Predisposing factors include lactation, diabetes, nipple piercing, and immunosuppressive therapy (13,14,15). Sinus tracts can develop following incision and drainage of an actinomycotic abscess, or with progression of the untreated lesion. A chronic abscess may form, creating a hard mass that can simulate carcinoma (13). Axillary lymph node enlargement generally reflects reaction to the mammary inflammatory process rather than spread of actinomycosis to the lymph nodes; however, primary actinomycotic axillary lymphadenitis has been reported (14,15). In advanced cases, the infection can spread to the chest wall. Extension of pulmonary actinomycosis to the breast has also been described (16). The diagnosis of actinomycosis is rendered by the demonstration of the Gram-positive organism in filaments or colonies (“sulfur” granules). Isolates from mammary actinomycosis include Actinomyces meyeri (17), A. viscosus (18), A. radingae, A. turicensis (15), and A. israelli (19). Treatment with penicillin has reportedly been effective (14), but recurrent or advanced infections may require multiple antibiotics (15) and rarely wide local excision or mastectomy.

Mycobacterium tuberculosis infection of the breast, in immunosuppressed as well as immunocompetent men and women, is not an uncommon condition in many regions of the world (20,21,22,23). Tuberculous mastitis has been reported as a manifestation of AIDS, and this presentation is encountered with increasing frequency in HIV-positive individuals (9,24).

Mammary tuberculosis unassociated with HIV infection is primarily a disease of premenopausal women with a predilection for the lactating breast, but it can affect the adult female breast at any age. Infection of the breast may be the primary manifestation of tuberculosis, but the breasts are probably infected secondarily in most patients even when the primary nonmammary focus remains clinically inapparent.

It is difficult to make a clinical diagnosis of tuberculous mastitis because the disease has multiple patterns of clinical presentation. The most common form is nodular mastitis in which the patient develops a slowly growing solitary mass. The mammographic presentation of these lesions resembles carcinoma (25,26). Microcalcifications are typically absent. Advanced nodular lesions become fixed to the skin and may develop draining sinuses. An acute and diffuse type of tuberculous mastitis is characterized by development of multiple painful nodules throughout the breast producing a pattern
that can clinically and mammographically mimic inflammatory carcinoma (26). A third, sclerosing variety of infection occurs predominantly in elderly women, resulting in diffuse induration of the breast and diffusely increased density on mammography. The clinical distinction between tuberculous mastitis and mammary carcinoma can be complicated by the occasional coexistence of both conditions (27). Rarely, tuberculosis of the chest wall can present as a breast lump (20).

Microscopically, granulomatous lesions in tuberculous mastitis do not always feature “caseous” necrosis, and fibrosis may be prominent in chronic cases. The granulomas are associated with ducts and lobules (Fig. 3.2). Acid-fast bacteria are histologically detected in fewer cases (Fig. 3.3). Neutrophils can obscure the granulomatous character of the process in specimens from patients with necrotizing abscesses or sinus tracts. Calcifications are uncommon. The finding of necrotizing granulomas in a needle core biopsy specimen may be considered presumptive evidence of mammary tuberculosis in the appropriate clinical setting. If mammary infection is suspected clinically, an aspirate or tissue sample should be submitted for microbiological culture or polymerase chain reaction (PCR) study.

A wide variety of atypical (nontuberculous) mycobacteria can infect breast tissue in acute, recurrent, and chronic forms (28) and can simulate a neoplasm (29). Breast abscess formation attributable to atypical mycobacterial infection such as Mycobacterium fortuitum (12,30) and M. abscessus (31) has been observed following nipple piercing. M. fortuitum infections have also complicated prosthetic breast implants (32).

The diagnosis of mammary mycobacterial and atypical (nontuberculous) mycobacterial infection and other granulomatous conditions is facilitated by PCR technique (21,28,33).